How to get large litters in mice/rats

[Auskan]

Just wondering what you do to get larger litters (if there's anything you can do). I have bred mice for years but was always going for fancy colors before, not quantity. Now that I am doing it for feeders, I'm more interested in large litters. I have 2 breeder females. One had 8 for her first litter (hasn't had the second yet) and the other had 5 & then 7. Is there anything I can do to increase their litter sizes? (I currently feed a combination of a home-made mix that contains mostly cereals, with a few sunflower seeds and raisins thrown in, some packaged "mouse/rat forumla food" that isn't that great because its full of stuff that they won't eat, and some dog food for extra protein. I'm also thinking about getting a bag of bird seed to add to their feed bowl. In addition, I occasionally (one a week or so) give them a bit of bread soaked in olive oil, which they love, and they also get the occasional fresh vegies as I have them).