Rather than being obese, that plump salamander may have severe edema. I have seen the same thing rarely in newborn toads. It would be an indication of an osmoregulatory problem and would likely limit the chances of survival for that individual.
You may be able to tell by "candling" the juvenile in a clear plastic container with a flashlight. Depending on the opacity of the skin, you should be able to tell if the "fat" areas are filled with translucent fluid, or full of more opaque fat or tissue.
-Tim