Lou weighs 31 lbs. at a little over 4 months. Don't worry about the size as long as they look healthy. Boxers aren't supposed to be chubby looking dogs, you should be able to see a couple of ribs at least. Their is no mention of weight in the AKC's boxer standard but other standards say that a dog should weigh around 30kg or so- somewhere around 60-70lbs and the females less. The boxer is supposed to be a medium (or smaller-large) sized dog, so don't worry if your dog doesn't get super big. If they are fed correctly, they'll end up where there genetics put them, looking at their parents is a good indicator of adult size. I'd be more worried if a dog was growing extremely fast as opposed to slower and steadier. If the vet says he's healthy, you shouldn't worry.