I really don't notice a lump when I feed my Bps. I do notice it with my boa, though.
As long as internal parasites are ruled out, I can't really think of a reason that a Bp won't get "fat" if it is eating regularly. In fact my largest male is looking rather obese, and i only feed him twice a month.
Your Bp does not look underweight, but not obese. I think that the real "fatty" Bps are the adults who are done growing in length and are just bulking up. I think that you could try a rat fuzzy now, one roughly equal to the size of a large mouse. I would feed fresh-killed, since it seems to me that rats are a tad more aggressive than mice.