My Maya was smallish and quiet and gained, though slowly. She was full term, though. My Maxwell was more than 2 lb. heavier at birth (also full term), but was slow to regain his birth weight and the pediatrician made kind of an issue of it- told me I had to feed him every 2 hours, and of course I did as he was my first baby.
The first thing that came to mind was what RCW said. The other thought I had was whether you had any IV fluids duirng delivery. I did with Max, did not with Maya, and noticed that even though he was bigger and more active and ate more and more frequently, it took him longer to regain his birth weight, and I wondered if it was because the excess fluids from the IV inflated his weight, if that makes sense.