I don’t think it’s silly at all. It is basically one machine with two screen sizes to choose from. I’m sure Purism figured out a way to do this efficiently or they wouldn’t do it. In addition to overall size, there are a lot of other options to choose from when you build your machine, including memory, operating system(s), etc.
Fwiw, I don’t like small screens either, but if you travel a lot and/or work during your commute via plane, bus, train or ferry, a smaller machine is really nice and it’s pretty easy to plug into an external monitor for more screen real estate when you’re doing something visual.