It's absolutely not restricted by the brain size. The brain size needs a huge head, so in a sense it is. All the factors are interwined. The growth of the bra

It's absolutely not restricted by the brain size. The brain size needs a huge head, so in a sense it is. All the factors are interwined. The growth of the brain seems to have been a very fast evolution, The pelvis had no time to adapt, so giving birth early was the final solution. With that in mind, I don't find fair to shame the pelvis, or the brain. The weakest link is the legs, but it's not their fault either since they had no time to mature. Give one of those cocky foals a head of proportional weight and allow them to use only two limbs and see who's stumbling now! Edit: newborns can't even balance their heads on their necks, they're extremely fragile.