They look a tiny bit small, but nothing severe. I'd add scraps around the house to your females diet. Making a cereal/dry noodle mix and feed them chicken bones, eggs, mealworms, high quality dog food, etc, will help. All of these are good for fattening and some are great for the protein, which mommy ratties need.

All in all, they look pretty chubby. If she had a large litter or she's a first time mommy, that could also be why they are on the smaller size.