It really does depend on the individual pup. My Hurricane was only 13-14 lbs at 4 months, he was 8 lbs at three months. Except when we first brought him home, he has never been really thin either. I think he is just going to be a small boxer. Even now, he is 9 months old and only 50-55 lbs. I think he is going to be on the small side for the males. If your pup is looking a healthy weight (not too skinny/ribs hips showing etc), and is eating, drinking, going potty, playing etc normally, and unless you have any other reason to suspect something is wrong, I wouldn't worry about it. Everyone told me Hurricane was too small for his age and he has been fine. Just my opinion, though if you are worried about him of course a vet visit is in order, of for nothing else than peace of mind!