[bd]

About that John Hawks's evolutionary argument - "if humans with smaller brains could manage, lesser energy requirements would select for this":

I wonder if maybe having larger brains didn't bring extra survival advantages thanks to having available "spare capacity".

From modern day sports we know how bad head injuries can be (boxing, American football, soccer). If in our evolutionary history a chance of brain damage was common, larger brains could be difference between "brain damaged but still functional" and "brain damaged and dead".

Interesting side-effect could then also be availability of this extra capacity for other functionality in individuals which managed to escape brain damage.