I don't quite understand this excuse. There are other characters that are smaller like Tracer (really small actually) and Li Ming. I would put Nova, Sylvanas, Valeera and Jaina in the normal size category and would have expected Alexstraza to be similar based on the WoW model. She isn't fat but bulky like Sonya. Alex not being a warrior should not be bulky.
The things that are more annoying to me are the furry cape she wears which makes her look like a hunchback and her really square jaw.