Actually, Olympus began the trend of smaller SLRs. Olympus often has been of the few Japanese companies that has dared to try different things. Nikon and Canon are sheeplike in adopting designs and then making changes.
Canon did make one big change when it abandoned its breech FD mount in favor of the bayonet EF. Aside from that, it's been one big sheep parade from Canon and Nikon.
Pentax has tried different things, notably the Pentax Auto 110 SLR, its "M" series, which some could argue are almost too small. My Pentax MX was my third 35mm camera, so I speak as a user and not a fondler.
I also agree that the East German Zeiss Ikon Contax created the SLR that set the visual pattern, except for its front-mounted shutter release.
The Zeiss Ikon Contaflex also was a solid SLR, although it lacked true interchangeable lenses and a rapid-return mirror. The Zeiss Ikon/Voigtlander Icarex probably was the ideal German camera and was the basis for one of the Rolleiflex SL 35 cameras (M and ME), as well as the Zeiss Ikon SL 706.
And of course, Exakta had been making its SLR for decades before they became popular.