Or perhaps the issue is that taller riders are forced to adopt a position that is more upright than smaller riders.

I say this half joking and half serious. Joking in that its could just be all about perspective. Serious in that for example- Japanese road bikes in the 70s and part of the 80s which had large frames were often built with top tubes that were seriously short, relatively speaking.
Geometry at extreme ends of the spectrum is often head scratching, even with the knowledge of history and using today's tech.