T.W.
As long as their height is in balance with their weight and their ped isn't concerned I wouldn't worry about it. My oldest daughter was 9lb4oz at birth, over 20lbs by around 6 mos, but she's almost 3 now and has only gained about 2lbs in the last year. She is big and tall for her age, but she's healthy. (My youngest was 8lb 11oz and is a peanut now...she's 15mos and still hasn't hit 20lbs yet). As long as their doctor isn't concerned I wouldnt' worry about it too much right now.