As mentioned earlier, the Zeiss ZM had a larger viewfinder. Zeiss was founded 3 years before Leica. Does that mean Leica should follow whatever Zeiss does because Zeiss has been doing it longer?
Personally, I prefer the Zeiss RF in every way. It can be harder to align your eye with the patch, but after some use it's not an issue. I imagine Leica doesn't offer different sizes because it would cost a lot to manufacture different sizes of top plates.
Having tried the larger ZM finder and the smaller Leica, I much prefer the compromise chosen by the Germans.
Having owned the camera with the largest eye relief offered (Ikon) and now an M240 I have to favour the M240 personally. I can appreciate that glasses wearers need plenty of eye relief but the Ikon was a pita until I got used to it.
In short ... Leica has it exactly right IMO.
Germans designed the ZM finder. The Cosina just built what Carl Zeiss AG (Headquarters, Oberkochen, Germany) told them to build.
So, which Germans were you referring to?