Yes you are correct, the volume inside human skulls are nearly all close to the same, you can make bones grow by breaking them and stretching them and letting new bone grow between the breaks, thats our latest tech and they need titanium frames etc to keep the bones aligned whilst its happening, but you cant make the brain grow to fill a bigger cranial volume we try to recreate, unless of course you are schlitze or beetlejuice who are born with microcefale?? (Spelling) or known as a pin head their brains are smaller and they end up handicapped and dont have control of theyre bodies like normal