beth is right. Some mice (especially the red colours) can be prone to obesity. However, I would check the food ingredients and analysis on the bag and make sure it does`nt contain peanuts, sunflower seeds and pumpkin seeds. These are all treats only and should be picked out of the actual mix.
If you also add in dog kibble, many can have a high fat content aswell, so unless you are breeding mice, I would`nt add kibble at all as a maintenance diet for mice does`nt require it. Dog kibble should only be a very small part of a diet anyway if you feed it at all.
If they eat all the food you give them, then your not over feeding. If there is food scattered around the next day, wait until they clear that before you top up. Fat mice are more prone to heart and kidney problems, so just make sure she`s not eating too many treats or her diet is`nt high in fat generally.