Xehanort's human form Heartless was born of an immensely strong will. There's really no argument for why it wouldn't take the form of the person that produced it. And actually, that technically is Terra's body. If you'll recall, Master Xehanort discarded his old body so he could implant his heart into Terra's younger, firmer body. As well, to pull off the stuff he does in game and cutscenes, Terra would pretty much have to be stacked. There's no argument that could explain him doing that and not being physically fit. Even magical augmentation is out, because if that were a possibility, why didn't Master Xehanort do it himself? After all, a master magician should be able to pull that kind of thing, right?