But I do want to ask, do your kids tip over?
My DS is tall and has a proportionate head, but he did tip over whenever we tried to get him to sit as a baby. He has a long torso and a skinny butt, so he had nothing to "anchor" him to the ground. In fact, "sitting independently" was never marked in the baby book because he really couldn't do until after he was walking! We'd set him down and then he would start screaming and then slowly lean to one side and then "plop."
OK, we didn't do it a lot, but it was funny in a sad kind of way. Body proportions definitely play a role in development.