Small is good! Thin is in!
It is so much better to have a dog who is a little thin then overweight. As long as your pup is otherwise healthy, don't worry about it!
As I stated before, obesity is the leading killer for dogs. 1 in every 5 American dogs is obese, with even more then then that qualifying as overweight.
I cannot emphasize enough times - PLEASE don't try and get your dog to add any pounds unless at the direct instruction of a vet. Please consult with your vet about this if you are worried. I am certain he will be the first to say - KEEP THEM THIN!!!