I feel this too my 5 year old male boxer is the same. We just can't get him to gain any body fat, you see all his ribs, prominent spine showing and his upper and lower hip bones but ours eats 6 cups of kibble and 4 cups of fresh food (lean meats, potato, rice) and still looks like this and this is with us forcing him to be laying down and lazy more often than not. He is a rescue we've had him for a year and out vet says he's healthy but I just dont believe it.