XFX 850W is 6.75" long. While the Seasonic M12D-850 is 6.25. Even tho as said they are basically the same power supply there are a couple differences. The M12D is listed as a as having dual 12v rails and comes with a 120mm Sanyo Denki fan, the XFX is listed as a single rail with a 135mm ADDA fan. I would guess that the larger fan is the reason for the extra depth.
Thank you very much Ancient_1 !