A lot of the brain just process information from the body and don't have anything to do with your conscious thought. People with larger bodies need larger brains, especially people with larger eyes but peoples eyes are mostly the same size.

Also what we think of as intelligence is mostly determined by personality. If your brain works 10% faster then average you are just going to spend about 9% less time thinking up the same mediocre idea that someone else with the same personality would until you think, good enough.