As has been said, it may well just be that she's still fairly young and not stopped growing herself. Did you watch as each baby came out? As it is possible that she had some stillborns, but 'disposed' of them before you counted up.
Just be sure to provide mum with lots of soft, easy to eat high protein food (especially as the kids start to wean, they'll need soft foods to get started on!).
Things to include in the diet in addition to the 'normal' stuff would be a variety of things, along the lines of fresh greens (curly kale is perfect), Naturediet Puppy food, Scrambed egg (or eggfood - the stuff you get for birds) etc. etc. This is an excellent site with a great food guide -
Shunamite rats Feeding for reproduction and growth
In the past when I fostered mums with litters, I followed the diet in the above website and they all grew up to be nice chunky rats
It's truly amazing to see the difference in size when you compare babies who have been reared on a good diet to those of a similar age in pet shops, who've likely just been reared on a 'standard' diet.