Anyway, for the SLRs, the viewer magnification depends on the focal of the lens attached : with wide-angles the image seems smaller, with the teleobjectives they are larger. With my Topcon, the magnification is 1:1 for my standard 58mm lens. It is also the case of my Spotmatic with the Zenit Helios44 (which focal is also 58mm).

Maybe the magnification is a little less than 1:1 on SLRs with 50mm lenses, because otherwise the viewing angle of the viewfinder would be to wide for comfortable viewing ?