Or is the small part the bit where the least useful stuff happens? ;-) A bit more seriously, the brain actually uses a significant chunk of the body's energy b

Or is the small part the bit where the least useful stuff happens? ;-) A bit more seriously, the brain actually uses a significant chunk of the body's energy budget (no matter how efficient it is relative to human made equipment). So evolutionarily, it doesn't make sense unless it's doing something exceptionally useful.