Sounds like the OP just has a fantastic feeder.
One of my girls gained about 100 grams a month and was about 500 grams at 5 months. She reached 1300 grams in a year. All my other girls who were born in the same month or a month older are half her size.
Not too fat! Obesity in any kind of animal is unhealthy.
And overweight breeding females are known to slug out more.