Wow, my '07 girl is over 500g, and she is fed every 7 days. I would suspect internal parasites if she is only gaining that little amount. Schedule an appointment with the vet and get her tested.

Did you get her April of this year? She could be younger than advertised. I know that when we got Sadis in August of 07, she weighed in at 90g. Now she weighs 517g. Thats 427g in 9 months.