To be fair, not everybody's reaction to stress is fasting. For many people, it's exactly the opposite- when their (inevitably bad) boyfriend dumps them (in some romantic comedy), they'll sit around watching older romantic movies (than the one they're undoubtedly in) and eating pint after pint of Ben & Jerry's (I wonder if they pay for that, or if it's free publicity). I'm not sure morphing would physically burn calories, either- it takes a great deal of mental concentration, but I get the feeling it returns you to the basic state you were in before morphing. You're mentally exhausted, but your body gets no benefit from it (except that your broken legs or wings or whatever are usually healed).
Dino's right, though- there has to be some kind of built in decision mechanism in the morphing technology itself that goes far beyond just DNA. Rachel morphing crocodile provides the perfect example. Crocodiles, like most amphibians, continue to grow over the course of their lifetime- when Rachel morphed the biggest croc in the habitat, she was also morphing the oldest. She wound up as the same croc, though- same size and everything. Morphing also apparently replicates coat patterns and that sort of thing on animals- it somehow manages to replicate the entire animal, but ignores injuries.
Maybe it recreates the animal (or original person) based on... super-Andalite-dark-magic (or, you know, science or whatever)... and then corrects anything that doesn't fit with the DNA? Hey, I kind of like that theory...
In any case, it almost seems like a given that a fat Animorph would remain fat after the morph- let's face it, morphing didn't continually alter the weight/muscle mass of the real Animorphs, so why should it for anyone else? It should, in theory, heal scars and acne.
Whatever complicated principles are at work behind the morphing technology, it certainly seemed to fit the story quite well, didn't it?
