yea a very healthy specimen is quite plump as you say. if you look at mine, the outer ring is inflated almost a 1/2" + over the bones. 2nd pic on the right side, you can see how deep down the bones stop. if its feeding, then keep offering it food twice a week until it gets better. also what lights are you running?
i just sort of named it the red and green open brain, not sure if it is on this site or not.