That is pretty small, are you sure of her age?
That aside, healthy is healthy. As long as she is getting the calories and nutrients that she needs, she will grow as she should.
Shadow is an average size female,62-65 lbs and at 12 weeks or so she weighed 17.5 lbs. But at 15-18 days she weighed just under a pound and at 6 weeks she only weighed 5 lbs. If your pup had a rough start she may take some time to catch up, or she may just be a little girl. Avoid overfeeding as this will not help, she is going to be exactly what she should.