It's unfortunate that the sensor size isn't even twice that of full frame, I have to wonder what it is that makes the body so much bigger when the sensor is only 10mm taller?
What's the buffer depth in RAW?
I guess it's a nice product for what it is, but I still don't see it being 3x better than a 5D3/D800.
You clearly don't have the faintest idea what you are talking about.
As has been mentioned look at the overhead view, a massive difference in terms of depth between the cameras.
The reason the 645D is so much deeper than FF dispite the sensor not being THAT much larger is I'd say because its having to deal with the legacy flange distance of the old "full frame" 645 system than a whoping 70mm.
Compare the 645D's depth to the Leica S2...
The formers sensor might be a little taller but that's a pretty massive difference which I'd guess is down to Leica having a purpose made mount with a much smaller flange distance.