Yes, jumbo is really just a retired breeder. I've had a few "Jumbo" strains of mice... mice that grew to huge proportions ... but they've always been sterile and almost hermaphroditic in nature ("balls" on either side of the vulvar opening). Since it was a homozygous sterile condition, I could never really produce them with any regularity. Other than that the only way to get "large" mice is to breed yellow ones, then you'll get FAT ones...but that sort of mouse isn't good for feeding on a consistent basis to snakes, AND they are so fat they can't breed for more than one or two litters.