I think it has more to do with how sheltered the children are. It is much easier to shelter a first child (or first children), then it is to shelter later ones. As the older ones grow they have more interaction with the outside world and bring that back to the home where it influences the younger ones. As a mashal, many oldest children do not get ‘nosh’ until they are older, whereas younger ones often have it very early, why? The older ones only know what they are given by their parents whereas the younger siblings see what their big brother/sister has and want the same.
I also find that oldest children are usually the most similar, personality-wise, to their parents, I think for the same reason.