I have one BP, my smallest and youngest normal female, that refuses to eat anything larger than an adult mouse. If it's a large adult or especially a jumbo, which she should be on now, she pulls back into her hide and totally ignores it. This, and the fact that she will randomly refuse prey, is why she's still so small and lightweight for her age (about 14 months).

I say, if they're eating regularly and the prey isn't REALLY small (like a hopper mouse), then let it be. That's just that snake's way.