Well, as for the regain of mass, I think that it is probably derived from z-space. We know that the excess mass for larger morphs has to come from somewhere (how else would they grow into, say, a rhino) and I think that it makes sense that it's drawn from the lack of mass in z-space. If all positive matter has an opposite equal of anti-matter, it would make sense that when anti-matter is drawn out of z-space, it would turn into positive matter, thusly augmenting your original mass. Least that makes sense to me lol