I have a theory, actually. This is a question to which I've given a lot of thought, and I think I finally have something.
Okay. Prepare yourselves for a science lecture that would do Ax proud. Here goes.
According to quantum physics, empty space is not actually empty. In any vaccuum, you will often get random matter particles which pop out of nowhere, accompanied, of course, by an equal number of anti-matter particles. This phenomenon isn't actually creating matter out of nothing, since the anti-matter basically acts as 'negative matter'. These particles never exist for very long, of course, since the anti-matter and the matter will promptly collide into one another and then cease to exist. Which is why you never see random matter flicker into existance in the real world. But, the important point here is, that you can actually get matter out of nothing, so long as you keep the matter and anti-matter separate.
So maybe, just as when you morph something small a blob of matter gets extruded into z-space, so when you morph something large, a blob of anti-matter gets extruded into z-space. As long as the anti-matter blob is kept separate from you during the morph, you can get matter out of 'nothing.' And then, when you demorph, the anti-matter merges back with your body, and the extra matter is lost.
This should theoretically work, since Z-space is big and empty, therefore the anti-matter blob has no matter to collide with and annihilate itself.
Heh, I am such a nerd.