I often heard about the sizing of preflop 3 bet, IP for 3x, OOP for 4x. This is also I'm follow. But could someone help to explain or send a link why we don't use 2.8x, 4.3x etc. ?

Is this because it's more easy for us to remember base on 100bb ?

If we have more then 100bb, like 125 or 160bb, do we need to change 3x to 3.3x maybe?

3x IP happens to be quite optimal for the most part by random chance

4x OOP tends to be ****

Got it, so OOP suggest use a 3-4x size?

You want some fold equity so you aren't constantly playing 3+ way flops... Doesn't make sense to just raise as a min click. Conversely, it doesn't make sense to raise to 20bb because OOP can just fold everything... Something in between (depending on stack depth) is likely going to be solid and that 3x sizing is sort of a sweet spot.

As for why we raise larger OOP it's generally because we are sort of forcing IP to play fewer hands and also creating a lower SPR (playing OOP at high SPR is really ****)... higher spr always favors IP player so in a way you're kind of chipping away a little bit more EV by utilizing a larger sizing here.

That is sort of my understanding, kind of hard to explain it more than this though, but that is my take.