I posted a thread here recently about my approx. 6 month old male ball python.
I got him from Petco, similar to Petsmart, and he was underfed too. I was very concerned.
So I began to feed him 2 adult mice every 5 days, simply because I wanted him to gain some mass--I was worried that as small as he was he didn't have the energy to digest properly.
I posted pictures from about a month ago of all the weight he has gained, and everyone said he looks fine, and not obese at all.
I wouldn't worry if it was underfed at first. It takes a little bit to gain the weight back, and once you notice it's plumping up a bit, cut back their feeding schedule a tad. (I.e. I was doing every 5 days, now it is every 7-8.)

Hope that helps. I can try to find a link to the thread where I posted pictures of my snake if you'd like.
Shellie