Milk really isn't the best for dogs or cats for that matter. They do not have the proper enzymes to break down the sugar (lactose) in milk. You've heard of lactose intolerance in people? The same kind of thing, people can be missing these digestive enzymes and not be able to digest the lactose, which can lead to diarreah.
Some dogs can tolerate a little milk every so often, some none at all.
On the weight issue, that can vary so much. As long as the hip and spine bones are not visible your pup should be fine. Puppies will go through stages of being real skinny and ribby, there are times when their skin seems like it is hanging off of them, then there are times when they seem a bit chunky.