I agree re. the essential similarity of the optical designs. However, the actual glass often accounts for a modest fraction of the overall lens weight. Having a leaf shutter (and in the case of the Rollei lenses, also those two linear motors driving the shutter and aperture) adds considerable weight, as do differences in the barrel and bayonet diameters and materials.
Agreed: differences like having a leaf shutter will have a big effect, but that would be the same whether it is a LS lens for 6x6 or 645. Lens mounts will have a difference, but in fact probably make the 6x6 lenses of the same focal length shorter and lighter, because the lens mount is further from the focal plane. (Not that this helps much, because the 6x6 body is larger and heavier by more than the lenses are smaller and lighter!)
Nope! I'm afraid you've seriously misinterpreted the "Area Covered" specification for all these RZ67 lenses.
Sorry! I grabbed the wrong numbers in my hasty search of the MamiyaLeaf web-site, where these lenses are now deprecated to "Legacy Products" with less information that before ... but I do recall seeing image circle specs that show the same trend, because the basic fact is that lenses from normal to moderate telephoto tend to use roughly the same optical designs, and cover about the same angular FOV before "format cropping" by lens hoods, internal baffles, film gates, and so on. Even a true telephoto lens design consists roughly of a front part that is a lens of a less long focal length with an integrated TC (diverging lens elements) at the back, which magnifies the image circle.
I have specs for the M645 lenses, and they follow the same trends under these headings.
Can you point me to the M645 specs? It would be nice to have some concrete examples of actual products, to go with data from an old textbook on lens design that I consult.