Prey = about the biggest round of your BP, is my vets formula, and works good with my BP. I feed rats, not mice. Don't know if there is a nutrition reason, but logically your BP will get to eating something bigger then a grown mouse, so go to rats. You can get a varity of size from what your using in mice now, on up to her need. Plus a little cheaper than 2 mice. Has your BP been treated for parasites? If it has them, they could keep its weight down and taking nutrition from it. I feed my BP once a week, and watch his growth according to he never gets so round his head looks to small for him, but his body is full, filled out and feels sound. Good luck. Joannarea