Baby weight for a 6 month old

Victoria
When my baby was born at 28 weeks, he was 6lbs 4 oz. Now he is 6 months and weights 13lb. He still fits in 3-6 months. 0- 3months are starting to get to tight. I'm just wondering if any one has had the same situation or similar? I'm scared my baby is not weighting enough for his age. Doctors say his fine, that he is just a small baby