I do apologize if this is the wrong forum, but it seemed most appropriate. I was wondering why the difference in sizing for road vs mountain bikes. For instance, I ride a 50cm (21") road bike frame, but I am supposed to ride a 15" mountain bike (38cm). Why the difference in fitting?

I am asking because I have seen an old (early 90's) rigid mountain bike for sale that I would like to convert into a hybrid sort of bike, but the frame is a 17.5" (which in theory would be too big). If using for road/path riding purposes, would this frame still be too big?