I'm quite convinced it's because she has absurd metabolism to feed that growing physique that is both tall and quite 'filled out'.
Being a human that shot up during puberty, as well, and has a resulting towering frame because of it, I understand quite well how 'tall person' metabolism is a magical thing that causes you to want to consume massive quantities of food, and yet somehow it all vanishes into your body with no sign it ever entered on your body, except maybe the temporary full stomach.