and is your skull size ruled by your genetics or do people have bigger heads because of better body and tongue posture?
I don't think skullsize and thickness is related to posture but genetics and nutrition?
Taller people tend to have thicker and larger skulls by circumference but maybe there large-skulled short folk.
Sure, mouthbreathing can be very detrimental to the splanchocranium by having a smaller mandible compared to what you should've had.
I don't really know what part correct posture and nutrition have in skull size, honestly.