I think, from that picture, she looks pretty normal to me.
As long as she is eating normally and she has been examined by a vet (as all puppies should be) then I would see no reason to worry. If she hasn't been to the vet in a little while, then perhaps you should submit a stool sample to your vet and have her checked for worms.
You need to keep in mind that Labs are a highly active breed; even for puppies. Sometimes they just run around and play so much that they really don't keep much weight on them until they get a little older and slow down. I know that Lexus stayed pretty lean throughout her puppyhood. She ate normally and everything but she just played and ran around SO much that she didn't keep a lot of extra weight on her.
As was said, you shouldn't be able to see her ribs. If you do then perhaps there is a problem that needs to be looked at. However, if you cannot see her ribs then I wouldn't worry about her size.