Only habitual things that you've already learned are done so mechanically. But you've already learned magnitudes more than your ice age ancestors as well as you

Only habitual things that you've already learned are done so mechanically. But you've already learned magnitudes more than your ice age ancestors as well as your pre-industrial and pre-information age ancestors. You've flexed parts of your brain and had to use more abstract knowledge than they ever had to. I suspect that some other factor is responsible for the shrinkage. It could just be pure sexual selection. Maybe we find smaller more symmetrical heads to be more attractive and we sexually selected for them over tens of thousands of years. Also could be responsible for domesticated animal brain shrinkage, as we select for more attractive animals. Could it be true that our brains got more complex even as they got smaller or did we legitimately lose some IQ points from the shrinkage?