Often the top half of the mouse looks fine, whilst a sprawling belly spreads out from underneath.
Obesity will shorten the mouse’s lifespan & make it more vulnerable to cancer, respiratory trouble etc. If you want to put your fat mouse on a diet, cut out all oily seeds (peanuts, sunflower seeds, millet etc), dairy products, fatty treats etc.
If possible, keep her on a diet of homemade mouse food (see above) and/or soaked staled bread, with lots of fruit and veg. You can give her vitamin supplements to make sure the change of diet doesn’t leave her with any deficiencies.