I just don't think that you can boil fit down to 1or 2 tube measurements. Back in the 70's and 80's , fit was all about seat tube length with little regard to top tube length. Then with Giant's introduction of "compact geometry" and sloping top tubes, we started caring about top tube length with little regard for effective seat tube length. Now, it sounds like some people are concentrating on head tube length with no regard for any other part of the front triangle. Then you have to consider handling and where the rider's center of gravity sits between the two wheels. As the bike gets taller, the rider moves further back over the rear wheel. The chainstays should be longer on big bikesthan small ones. Giant keeps 420 chainstays on all of the sizes posted above. Longer chainstays aren't fashionable though. I really don't think that proper fit or handling is a big priority for the big bike companies today.