I have been pondering the same thought and have come to the conclusion that one small rat compared to multiple mice would have to be less skin and bone percentage. But absolutely no proof or studies but just my thinking, hey that's alot of skin and bones and not much meat or muscle with mice. But that's just my thinking. Plus the other reasons stated for buying rat instead. I like keeping the rat on the small side and not getting too big a one.
Also you said 3 years old and 3 ft. I think it's possible you have a male there instead of a female, lets see what some others say about it.