... It's worth noting that the actual area of the AF point on the AF sensor is larger than the little box in the viewfinder. Spot AF uses a smaller area of the AF point, but even that is slightly larger than the box in the VF (the main box, not the smaller inset box that indicates Spot AF).
This is something I have been loathe of forever! Why camera don't make the AF-markings exactly correspond to the true size of the real AF-Sensors extension? Probably to cover up for all that misalignment inherent with the very design of mirrorslapper cameras. Moving mirrors and submirrors pl,us a number of separate planes ... AF-sensor, imaging-sensor, mattescreen, transmissive LCD, viewfinder image ... with a big fat prism (or a mirrored hollow space) in between.
But even in mirrorless cameras, I find AF-field markings WAY smaller than the actual AF-sensor. e.g. in the EOS-M. It irks me all the time, since it contributes to the blasted thing focusing on contrasty fecnces in the background rather than on soft faces in the foreground.