What is the cause of my VERY obese pet mouse with hair loss on face?

I have two pet mice, the silky colored kind, which I got at PetsMart, unfortunately. They eat a diet of normal mixed mouse food, with seeds and so forth, and I just fill up their dish when it gets low. Well my issue is that one mouse is very thin and looks like a normal mouse, and the other grew to be probably five times it's size very quickly after I got them (they were babies). Also, after about six months of owning them I recently noticed that my fat mouse has lost all the hair on her face, but her body hair appears unaffected. What could be causing this sudden loss, and any ideas on why one grew so enormous when they have the exact same lifestyle? I know every living being is different and some pets tend to become overweight more easily, but I have owned rodents and bigger animals my whole life and have never come across anything like this! It is seriously ridiculous to look at them side by side.What is the cause of my VERY obese pet mouse with hair loss on face?
I had a pet mouse named Liversnap that was very chubby even though he ran on his exercise wheel pretty much every waking moment, and he lived longer than any of my mice. Your mouse might just be like that. However, it could be something more serious.I really don't mean to scare you, and this is most likely not the case, but pet rodents, like mice and rats, are prone to getting tumors, especially if they are mixed breeds. Luckily, although some tumors will eventually cause death, many are benign, or at least very slow in growth. I had a mouse named Pikachu who had a tumor that was very large, almost the size of a smaller mouse. She, however, managed to live a full life, and was only outlived by two of our mice, out of all 23 of them. Another one, named Chessie, managed to survive two pregnancies despite her tumors, that were internal and not noticed until she was near the end of her life. As for the hair loss, I can only suggest that she may have mites, or that she is getting ';barbered'; by your other mouse. Barbering is when an animal chews on its own fur or another animal's fur because it is bored. I would check with a vet or a book about it.