I'm not a lens physic expert but I disagree with the tube diameter being irrelevant to light transmission.

I did a Google search on telescopes and lens size. It's Always mentioned the larger the lens the more light is gathered. While size of the telescope is mentioned, width etc, it's not in a brief reading mentioned if subsequent lens are relevant or not. But I can't see how the first is relevant and later lenses are not. These giant telescopes have pretty much straight tubes, unlike a rifle scope. I'm unaware of the constructin inside these straight tubed telescopes.

Secondly hunting spotting scopes. Why are their designs similar to a telescope and not a hunting scope? More or less straight tubed spotting/telescopes. Increased magnification to a hunting scope, but need always increased light transmission. They don't need the fine precise movements of a reticle.