Can someone give me an idea on why 3 of my females babies keep getting smaller?

I agree with allanis* my rat has babies and for some reason they come out so cute and great marking but smaller then a normal rat! IRS a mutant gene. Teacup mice n rats LOL. I sell mines at $23 for one an still in demand. But I don't over breed my rats I'm hoping her sisters carry the mutant gene