She looks to be of good body weight from what I can see. Feeding mice or rats really doesn't matter, what matters is the care they are receiving and if their environment is appropriate to allow them to thrive and grow. If hers wasn't, then she wouldn't be at her full potential.

I have a ball that I rescued when she was around 4 years old. She was only 780g but had good muscle tone. She was only being fed one small mouse a month. She gained over 1500g from May 06 to December 06 on 3-4 mice a week and hasn't quit eating since She's now around 2200g and doing just fine on 3 medium adult mice a week.

Just feed her appropriately sized rodents every week and she should gain some weight. Some females are built small though, as genetics will determine that and not food consumption.