From everything I've read and seen females typically grow larger and at a slightly accelerated rate than males. It's typical of almost all species.

Have you been keeping track of his weight weekly at all? It might be interesting for you to see exactly how much weight he's gaining each week. Once you've done it for a few weeks you can work out a sort of trend, so long as it's consistent I wouldn't worry too much. We used to do this with underweight dogs at the vet clinic, it's all about just finding a happy medium, rapid weight gain can be just as dangerous as being under weight for animals.