^ TBH, I actually kinda assumed it was the opposite - that the taller one is very big for her age. Or just much taller that the human woman's average in general
If I remember right from other pics on their Pixiv, it's a little of both: Big girl is huge for her age, Small girl is tiny for her age. Like basketballer tall vs gradeschooler short. Put them together and it enhances the gap.