Utilising the mantra 'small hand, small pot and big hand, big pot' if I have a second pair type hand, I want to play a small pot.

I see a small pot hand as a hand that can get one street of value post flop. Two streets of value is a medium pot and is a pot size that is too big for the strength of a second pair type hand.

It is 6 handed. Now I have JcJs and I 2bet from MP to 2.5bb. The BTN calls and everyone else dealt in the hand folds. There is no rake in this game so the pot size is 6.5bb.

The flop comes 9c,6d,4h. I continuation bet 4bb and the BTN calls.

The pot size is now 14.5bb.

The turn is Kh so the board now reads 9c,6d,4h,Kh.

Now what do I do here? My hand has turned into a 'second pair' type hand, so the pot size has already been achieved by myself.

If I bet the turn, I feel like I am overplaying my hand and building the pot too big as if I am seeking a second street of value from my opponent, which my hand is not strong enough to get.

If I check/call the turn, I feel like I am overplaying my hand as again the pot size is growing larger than my hand warrants and I am only really happy if one of the two remaining Jacks land on the river, but that would be very rare.

So therefore by using the 'process of elimination' method, I don't want to bet, I don't want to check and call, so therefore I must have to check and fold.

Is this the correct play for me to make each and every time this kind of thing occurs, using the rationale described above? It seems like I am sacrificing the pot a hell of a lot, and I am vulnerable to my opponent floating my flop continuation bet, but then maybe because I am OOP this is ok?

Overcard turns can be tricky. We have to consider range Asymmetry - sometimes we want to RB small, sometimes blocking w/ a polar range, sometimes over betting.
Fwiw flop is RC and probably mixing XR w/ your holding and XC AA, KK more so. BU flat is extremely condensed and being OOP we can often apply "pinnocks razor" and just check range. Grab yourself a solver and look at Overcard turns.

I don't like check/raising with any overpairs here, as they are just one pair hands and so these hands want to play a medium sized pot come showdown. This can be achieved with check/call, check/call, check/fold for example.

Raising the flop starts making the pot get very big, very fast and does not seem optimal as perhaps it folds out my opponent's bluffs and forces him to continue only with hands that beat me, so in effect I value-own myself.

Hard rules like your second pair one in relationship to the size of the pot are not good. His range barely contains any Kx after calling a 2/3 bet on flop and he is going to raise flop with a good chunk of sets so you end up on turn with the best hand the majority of the time. You can barrel turn and river small for example, I like this better than checking.

The problem that I have with betting small, is that we probably wouldn't be taking this line with a big pot hand would we? This sounds like a great bluff raise spot for villain. Also, why can't the villain have all sets here after flatting the rainbow flop? He even could float with all his Kx with a backdoor flush draw. I agree that the villain won't have loads of these hands and so the turn has a good chance of checking through in that case right, which for our hand is preferable than us betting small, so we should check and then 'see' if it checks through?

Also, does this hand change much if the flop is K,6,4 rainbow and the 9 comes on the turn? I guess we would check/call the flop in this scenario, but we would then have to check/fold the the same size pot of 14.5bb on the turn because the pot is threatening to get too big?

Finally, you say that rules about building the pot size to your hand strength are 'not good', but you don't really explain why. I thought that was the goal of poker, and that a big mistake fish make is that they build big pots with small and medium hands.

Rules are there for fish to not punt, which is fine. Good players should strive to understand each spot individually.
This is a very simple example:

BTN vs BB Single raised pot: Flop is 9c5s2h
You have JJ, suits don't matter, and you bet 200% the pot, villain calls, turn comes Q.
Villain has very few to no Q, and our hand has probably 75%+ equity. You can bet this hand.

Now rewind back. 9c5s2s. You bet 25% of the pot, villain calls, turn comes an offsuit Ace.
You have second pair in both scenarios, and in the first one the pot is bigger than the second one, but here, you have significantly lower equity because villain is going to call flop with a lot of Ax hands. Betting here is much worse, even though your absolute hand strength is the exact same and the pot is much smaller.

Firstly these examples are not the same as the turn card was a Q initially and then it changed to an A.

The other thing is that in your first example, betting 200% pot maybe forces your opponent to only call with 99, 55, 22 and perhaps 95s if he has it. Against this range, JJ does not have 75%+ equity so you shouldn't bet it again on any turn card other than a Jack. It also seems like you have turned your hand into some kind of weird bluff as you are building the pot size bigger than a single pair of Jacks deserves to build it.

It is interesting though and as a side point with your first example, if you open shove JJ on the flop for 97.5bb after the BB checks, I am guessing that it is guaranteed +EV for us, but it is bad because it makes our opponent's decisions easy where he just calls with his two pairs and sets, and as a result it is not max +EV for us correct?

From what I can read, your rule of thumb is decent, but like any rule of thumb, it is a simplification. As you progress, you should be aiming for your simplifications to be less and less simple. Some more important concepts might be relative hand strength, or how your hand fits into your range, how your hand compares to villain's range, how your hand compares to villain's continue (call or raise) range, and how your range compares to villain's continue range. I think the first step is comparing your hand to villain's range.

In your example, what type of hands do you expect villain to have called pre and called flop? Of those, which ones would you expect to call a turn bet? Which ones do you expect to raise a turn bet? What do you expect villain's range to look like if you bet turn then check river? If you bet turn then bet river.....?

Some simple questions to ask yourself at a decision point are: Do you feel like you will get called by a decent proportion of weaker hands? If yes, you can consider (but shouldn't always) value bet. Do you feel like you will fold out a decent proportion of better hands? If so you can consider bluffing. Do you feel like if you check, you will be ahead of villain's bluffs but not their value? Also, do you feel like villain will bluff at least at a decent amount? If so, you can consider bluff catching. I think classifying your hands in those three categories (value bets, bluffs, and bluff catchers) plus give ups may be more helpful to you. Obviously, this classification needs to be updated on every street.

?

that is the whole point of that example- that you can't treat all "I have second pair" situations equally.

As for the first one if your opponent is only defending 2pair+ vs. a 200% flop bet he is massively overfolding. That's 11 combos of hands (depending on preflop frequencies with 99/95s, so in actuality it is even less), and I don't know the exact numbers but lets just say you call 30% of your range preflop, which is 1326*.3 or roughly 400 combos.

So you're folding something like 97% of the time when your opponent bets 2x pot? That is most certainly not correct. Try to think of poker in terms of ranges instead

The way I see it, either a hand is profitable to call OOP pre-flop or it isn't. No crazy betting pattern or sizes from my opponent can 'make' the hand a bad call.

Therefore if my range is comprised entirely of +EV hands that have a +EV call of 2bet pre-flop, then my opponent jamming the flop huge shouldn't change anything for me right? I just look at the odds I am being offered and continue with the hands that it is profitable for me to do so. If I have to overfold on some boards due to not many of my hands connecting and my opponent betting huge, then so be it. If he is doing it with JJ on a low flop, then look at how much value he is losing, it's a crazy bad play.

This is absolutely wrong, the EV of preflop actions is very strongly affected by how both players play the hand postflop. Villains crazy betting pattern on the flop could turn a GTO +EV defend into -EV, and if you have such a read you should fold it.

Going back to the example of JJ, I'm right and ur wrong basically. lol. JJ betting 200% on the flop is a theory approved play and you can calculate it with a solver, while folding all but sets and 2 pair is definitely not.

How can a GTO +EV defend exist, but against an unbalanced opponent they can make the hand -EV? I thought the whole point of GTO was that it was unexploitable so your opponent can't make you -EV in spots that are theoretically +EV?

As for betting 200% pot with JJ, it is scared poker, not wanting to get outdrawn on the turn, but I learned that you can't protect a vulnerable hand by betting a tonne of money, poker doesn't work that way.

I saw a solver video the other day, and one of the worst things that the player could do, (i.e. least EV), was going all-in on the flop in a large SPR pot.

Also, did you see that Helmuth vs Polk hand from not too long ago? Helmuth went all in on the flop with a huge raise, and Polk correctly calculated that he only needed to really defend the nuts so he folded the second nut straight. Against your 200% pot bet, you don't need to just call with the nuts, you can call with 2nd nuts, 3rd nuts and 4th nuts too.

I would just show you the solver choosing the 200% bet but ud prolly just dunning kruger ur way out of learning anything new

As for the +EV turning into -EV thing, yes, it happens all the time. GTO being unexploitable doesn't mean it can't make -EV plays against certain players, for instance, GTO will call you with some hands on the river even if you never ever bluff, making those calls -EV.
This can happen preflop aswell, for instance, an human can be so extremely aggressive postflop that the weakest hands in GTO's ranges lose money by defending preflop. GTO is still unexploitable and beats this particular player by virtue of having its strongest hands in range be much higher EV than in equilibrium. But, it would still make more money if it just folded the bottom of range preflop.

To be fair, I have seen people say that changing your bet size doesn't really change your EV in any meaningful way.

I take this to mean that if you bet more, you give your opponent worse odds to continue so he can correctly fold more of his range. Likewise if you bet small, your opponent has to continue much more frequently. This is also complicated even more by the fact that you have to look at all possible boards as a whole, as your opponent does not need to defend all boards at the same frequency, instead he overfolds on bad boards and overcontinues on good boards.

Another way I like to look at things is to forget about my range completely, and instead just look at my specific hand and see how it is doing, for example QTo on the 9,5,3 flop. I can profitably fold it on this board to your 200% pot bet and not lose any sleep at all, as I know that there are other boards that I can defend this hand on.

This is interesting and I think I agree.

So if someone overbets a lot post-flop, it is likely to make small pocket pairs ++EV and a mediocre hand like Q8s might be -EV now, whereas if your opponent keeps his bet sizing within the size of the pot at all times, the small pocket pairs and Q8s are now both just slightly +EV.

Is this what you mean?

It's important to realize that GTO aims to maximize the EV of its entire strategy, rather than every particular line. This means any of your imbalances (e.g. too value heavy, or too passive, or too aggro, etc) will print vs some part of a GTO strategy at the expense of the rest of your strategy.