Being big pays off every day and is punished in extinctions, which is why body size keeps getting reset rather than climbing forever.

You're dominant in contests, particularly with conspecifics. If you're trying to win a territory or win mating rights, bigger things usually beat up smaller things. So there's going to be selection favoring them. But then big things don't usually do well in extinction events, so that tends to reset the clock by killing off the big stuff, and then smaller stuff does better again.