Well, they found out not very long ago that very smart children's brains grow faster than other brains, but then prune back more. Maybe they need to prune back so they can fit.
I assume they must prune back less important stuff because the kids grow up to be very smart adults.
Occasionally they discover that ordinary people have skulls filled mostly with fluid, their brain matter severely compressed. Do we really know essentially how the brain "produces" thought?