It's difficult to get across how rapidly the human brain enlarged. From our ape-like ancestors to the bigger-brained humans was two and a half million years. That may seem a long time, but in evolutionary terms, it's remarkably quick.
The human brain was increasing by the equivalent of 150,000 nerve cells at each generation. But in nature, there's no such thing as a free lunch. Everything has its cost, and a bigger brain, like a bigger engine, is more expensive to run.