I would think that the reason he has a fat belly is because he has just ate and there is a lump from the mouse.
I think we would all like to see some pics. Rats/mice don't usually take up that much space in a freezer, let alone pinks. I ordered 40 rat pups from rodentpro, and they barely take up any space at all!! Oh, and if you are going to start ordering, I recommend Rodent Pro.