As it says in the paper, the evolution of hominids doesn't look like this; population genetics says that if hominid brains got larger, the marginal fitness returns on neurons went up.
Or in a more intuitive sense, humans didn't seem to need brains 1000x as large to get one unit of improvement in practical effectiveness over chimpanzee cognition. But yes, this stuff is complicated, hence the paper not being two paragraphs long.
You've probably read Scott Aaronson's Why Philosophers Should Care About Computational Complexity? [0]. This seems like the perfect area to apply a lot of the questions he brings up. That's what I was looking for as I skimmed through the paper. Maybe that's what idlewords was talking about as well.