Don`t know how long she had been living with her aunt and uncle but poor nutition could also be a cause. It didn't sound like they were treating her well.
IRL, I have quite a few friends that are short enough they can sometimes get a child's fare still. Tallness is recessive and harder on the heart after a certain point. And quite a few actors are smaller than they appear on camera. My friends and I went to the Harry Potter exhibit when it was traveling around and Emma Watson's clothes wouldn't have fit any of us after we hit puberty. And the lady who played Lestrange was super tiny too.

Why is she so short and small, she is not 12, I refuse to believe that