Like humans, I guess boxers come in all shapes and sizes!
Biff was a skinny youth, but at 16 months, he is still filling out, and I think he looks perfect at the moment. Still lean, but getting quite muscular. Buttons is only 9 weeks, and a right skinny wee runt
A lot of it will be down to diet and exercise (again just like humans
And there again, I know quite a few people who eat like horses but never gain weight - and the others who put on pounds just walking past the cake shop
As long as you know your boxer is getting a good diet, and lots of exercise, and is fit and healthy, there is no need to worry.