yeah I'm kind of with the OP on this. I mean I agree with the consensus that there is random particles of matter or anti-matter or potential matter or whatever floating around in Z-Space which gives our intrepid heroes the ability to morph into something of greater mass, but the fact is, when they morph something of smaller mass, their own unique mass is sent to Zero-Space completely distinct from all of that other mass. Remember in 18, when all the Andalite scientists were floored that the extruded mass of the Animorphs had shape, pattern, cohesion? That they could be reintegrated into normal space from n-dimensional space? That implies that the base morph, or neutral form or whatever is considered one distinct unit to the morphing technology. If Jake got his arm chopped off, that would no longer be a part of the unit, so what would PROBABLY happen, at least my theory, is that the code from Jake's DNA would just make Jake like it should--complete, with two arms--but only using the mass he extruded into Zero-Space (minus an arm). So he'd be Jake, but roughly 10 pounds lighter.
I mean, obviously this theory is entirely contingent on which book you're taking as canon. My theory's based on #18, just because that's the one where I feel like we get the most hard info about the morphing technology, but like someone said, #32 contradicts it, I'm sure there's a book where Ax loses his tail or leg or a bunch of blood or something, idk.
In either case it's a fast way to lose weight, right? Wanna lose forty pounds, cut off a leg. If only you could guarantee the morphing technology would consider your fat less relevant to construct than something else, like your liver or heart.