In an SLR, the apparent magnification depends on the lenses between the prism and your eyes (viewfinder magnification) as well as the objective lens' focal length; for a given viewfinder magnification, you will obtain a 1:1 view at some objective particular focal length. You can get a magnifying eyecup that clips over the back of the prism and gives you (typically 1.4x) greater magnification - I have the Pentax version installed on my Sony DSLR that makes manual focusing a little easier though of course at the cost of a dimmer image.

There are plenty of SLRs with 100% viewfinder coverage; it's just that they're a bit annoying to build because it requires an oversize mirror; there is nothing special about the mirror that means you need to have a reduced view. A bunch of Nikon Fs, as well as Minolta 9 and Sony A900.