The A7R II has a BSI sensor so it probably collects a bit more light per area. The D810 has the ISO 64 which too works to its advantage when there is enough light to use it.
When I was shooting 35mm film in the 1990s, I looked at images printed from 6x4,5cm film, 6x6cm , 6x7cm , 4x5 inches and 8x10 inches. I felt the 6x4,5 was a bit better than 35mm but to get really blown away, 6x7 cm and bigger did that. I got a 6x7 rangefinder and shot with that for a few years, and yes, the image quality was great, the detail in print was amazing once properly scanned (LS-9000 in my case), but most lenses were designed with compactness in mind and I wasn't that happy with the relatively small apertures. The 6x7cm diagonal is sqrt(56*56+72*72) = 91mm vs. 43mm for FX; the ratio between the diagonals of the two formats is 2.1:1. For the 33x44mm sensor the diagonal is 55mm; ratio between the diagonals of the two formats 1.28:1. So the sensor sizes are not different enough to make a really clear difference in image quality especially given the variation in lens characteristics in each system. Yes, the images shot with the larger sensor are probably a little better, but I would consider the system as a whole and see if it offers features which together with the larger sensor size make it worthwhile.
For example, the X1D has the leaf shutter lenses which are useful for outdoor portraits with flash in particular, the GFX has a tilting EVF which I would imagine would be very useful for some kinds of technical photography and, e.g., shooting portraits in sunlight if you want the camera to be at waist level without kneeling down; the D810 and 645Z have optical viewfinders etc. All these differences make the systems different in what you can do with them even though image quality differences are not that large. It is possible that Fuji, Hasselblad and Pentax make 100MP variants of the camera bodies in the near future; perhaps the differences will be more pronounced at that point.