Also, some of our pups are just generally of a different build to others...
Harry weighs 20lbs (he's 3 years old in October) - but you'd never think it. He's bigger and much slimmer - even at his weight I can feel his spine when I run my hand along it.
When he's in the bath, he looks like one of those pups that you'd try and save from a Charity advert....
I homecook for him - and keep Royal Canin adult down all the time for him to graze on....but with him he kind of self-regulates - I find he eats more in the evening, but that's just him.
I totally agree with Kathy - I'd get my vet to check your baby over and see what he suggests. Good luck!
Sally + Harry x