So brain power and strength are some how linked? That makes sense on why people think most muscleheads are stupid
Well there's not a causal relationship there, meaning bigger muscles doesn't actually cause you to be dumber. In fact, the physical activity you put forth (weightlifting) to acquire the muscles will release a growth factor BDNF in the brain which will cause new brain cells to grow, facilitate brain plasticity (development of neurological networks) and consequently make you smarter. So, lifting actually makes you mentally more capable, not the other way around.
When we look at the difference between humans and apes we're going to have to account for
mutation and mutation only. Somewhere our breed got bigger brains through mutation and relied on on them more and more through time. Therefore, the bigger the brain you got the smarter you were and since we relied on our smarts to survive the ones that survived were the ones with the bigger brains and ultimately we arrive at our current form, a form that is likely heavily influenced by the genetic manipulation of and possessing genetic relation to extraterrestrial lifeforms that are yet more advanced than--yes--our perfect selves.