Does he eat well? What type of food does he eat? Do you free feed or feed meals so that you know how much he eats? I think the protein content for puppies is very important in their growth and development. What is the protein content of his food?
Rowdy was/is a very picky eater and he is very slim - just over 4 lbs at 14 months. Rexy, on the other hand, may eat us out of house and home!
Rowdy is just not interested in food, whereas Rexy LOVES to eat.
How about your baby? Does he love to eat or is he picky? I think their natural tendencies toward food may contribute to their size.