Mega colon, maybe? I can't tell very well but the one looks to be female and could be pregnant to explain why she's that big. That's crazy that those are MICE! Look like small rats at that size, lol! Almost look like retired breeders from the size/condition of their bodies/fur. I don't know on that one, sorry. I breed Norways and have no experience with mice other than the occasional purchase of them as feeders. Good luck and hopefully someone else chimes in.