I wouldn't be worried.

I think they do a lot of growing in the first 4-5 months. Jackson weighed 5lbs when I brought him home at 9 weeks old, then was 6.5lbs at 12 weeks old, then was 8.6lbs at 15 weeks old, and by around 5 months old, he was 12lbs. Now, his adult weight ranges between 15-16lbs.