If you take a look at Harrington on Hold 'Em Volume 1, published just before 2005, you'll notice that in it Dan says "I like my continuation bets to be about half the size of the pot" and "...half-pot is an ideal size...". He goes on to say you can vary your c-bets from around 40% pot on the low side (for dryer flops) to 70% pot on the high side (for the very, very drawy flops). He appeared to believe half the pot was enough to deny proper odds even on something like a J82hh board

It seems to me the average size of a continuation bet has gradually gotten larger and larger since then. Even by 2006 I think half-pot was already considered a rather undersized bet and now, by 2008, it certainly is. Pot-sized bets aren't anything out of the ordinary and one could argue that the average c-bet is now something like 3/4 pot, which used to be considered on "the high side".

Is there an actual theoretical explanation for why the bets have gotten larger that Harrington was missing out on with his 1/2 pot bets? Do people find it narrows ranges and makes hands easier to play? Or is it simply a case of.. well, other people started betting bigger, so in order for my bet to be as effective, I must bet bigger as well?

it makes it harder to play against, because floating / bluffing costs more...

besides harrington didn't point out that you have to protect against implied odds too, especialy if you get floated a lot.

People have stopped playing very badly, & folding too much to half pot cbets.

Many up and coming stars in this game are LAG...and their style is being imitated by a lot of people....so overbetting has become quite the regular play in holdem now (it may even be overused imo)

Might be because he wrote about tournaments which usually have stacks way smaller than 100BB, and implied odds are a lot lower. Also people fold a lot more in tournament to protect their stack.

This is why his cash game books need to be out already.

Harrington is a god of large tournaments, where you need to maintain your stack and rarly have 100 times the bb.