my ds was 3520grams at birth
he dropped 390 grams
his discharge weight was 3910 grams
and he was 51 cm
at 2 weeks he was 4180 grams and 55.5 cm
at 6 weeks he was 60 cms and 6000grams
at 6 months he was 75 cm and weighed in at 9.57kgs.

he is now no bigger then any other child his age.
I wouldnt worry, my ds loved the boob and just flurished on it.