23fbaf81_ess___Photographically_Curious__Mirrorless
[Overall] Size and coverage [SLR] Only the best bodies have large viewfinders and 100% coverage. Low-spec bodies (especially low-spec APS-C bodies) have low magnifications (resulting in a smaller viewing area) and less than 100% coverage (so you get more in the image than is visible in the viewfinder). [Mirrorless] 
All bodies have 100% coverage. Higher-end bodies have very large viewfinders (even larger than some full-frame OVFs).
SLR vs. Mirrorless | Photographically Curious
Mirrorless