What would Hitler be like with a larger brain?
That much more insane? And look at what a large head our Chief Clown in the Whitehouse has.
Large brains do not determine intellegence alone. We would be working for elephants otherwise.
What do you have against bigger brains?
Neanderthal survived In Europe during the ice age with nothing beyond stone tools. During the same time CroMagnon/Homo Sapien did not venture much beyond the 30th latitude. They must have had something going for them.