Buster's weight sounds fine for his age; Boxers grow til they're about 2 yrs. of age. Ruby is 2 1/2 years old and she weighs 52 lbs. She didn't really fill out though until she was about 18 months old.
Ruby is also from champion blood lines and from the few champions I've been around, they seem smaller than you would expect. I get comments all the time about Ruby like "oh, isn't she small?" and "I always thought Boxers were bigger". My husband and I thought Ruby must have been the runt but when we went to the shows and looked at the ones in the ring, she's the same size.
A few times when I tried to make her "fill out" more by feeding her a little extra food, it seems to have given her a small cow-like udder instead of actual muscle and bulk. It seems as though there is a fine line with them between keeping them lean and making them fat.
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-Elsa
Ruby- Flashy Fawn, Black Mask,Cropped, 5/1998