Then what I would do. Is feed 10% of they're weight. Once a week. Up until they can handle small rats. Which are 45 to 65 grams. Only because I feed rats. Or in your case about 2 or 3 adult mice.

But I'd say 2 mice every 5 days should do good to about 250 to 300 grams. Then back off to every 7 days. But still feeding about 10% of they're weight.

You ever have a fecal done on the smaller one to rule out any parasites? That could be keeping her smaller.