Conjecture here... Maybe the super-aggressive fish of old got caught and harvested (as was more common then), leaving the lighter biters. Or, even if those aggressive fish were released, they learned to be a little more tentative with their strikes after being caught? I've seen in just a year or two of fishing smaller waters, how fishing pressure will push the bass to feeding at unusual times....after dark, for example. Magazine articles have been written about such phenomena.
Anybody else seen similar behaviors?
Also, what about.... perhaps when we select for big deer and big fish (for example), it leaves the smaller ones to pass on their genes, creating a loop of ever-smaller game? 10" Bluegills aren't nearly as common now.