Wow, Hudsen was 30lbs (approximately-he wouldn't sit on the scale) at 16 weeks. I just weighed him the other day and he is 40lbs and 21 weeks. I kept thinking he was underweight because he looks so thin.
What you should be aware of, is that the weights mentioned are following Slowgrow. If you don't follow that feeding and exercise plan, the weights are merely an indication, nothing more. If your vet is not complaining, I wouldn't worry. Tess is 13 weeks and about 21 lbs, she looks skinny, too.
IMO, at this age they seem to look "skinny" because they are starting to grow out of the roly-poly puppy stage. And, they all have growth spurts at different times. If your girl is healthy and active, I wouldn't worry.
There also seems to be a difference between the males & females. The growth charts don't allow for that but looking at the different threads here on the forum, that seems to be the case.