Someone back along in the posts said that in order to provide FF coverage the lanses had to be big - not at all. I was lucky enough to be an ambassador for Hasselblad, and had an X-pan given to me to use with 30mm, 45mm and 90mm lenses. I know absolutely NOTHING about lens design. Much of what you lot have been arguing about has gone right over my head because I don't have a clue what you were talking about. I am and have been a photographer, not a lens designer. All I knew was what kind of picture a certain lens produced and I went out and made pictures - I didn't need to know about retro focus (whatever that is - I didn't know my Nikon lenses with the D4s had it!!)
What I do know is, the X-pan was no bigger than my FE2 or FA, in body size, but without the great prism on the top. The 90mm lens was about the same size as an Actimel bottle, maybe a little smaller....the 45 lens was, predictably, half as long physically as well as focal length wise. The 30 was no bigger than the wide angle adaptor for my Coolpix P7800 (which I still love and have a full bleed DPS in Saltwater Boat Angling this current month from, I have had several front covers from it too). Now, the crux of me saying about the size of these lenses, with regard to the Full Frame argument is this - those lenses had the image circle to cover a 6x7 frame, because the X-pan used 35mm film to produce a masked off panoramic picture from a 6x7, instead of throwing away a slice of roll film from each side of the mask (RZ had a panoramic mask adaptor film carrier to allow the use of 35mm film through the monster body).
If those lenses can cover TWICE the image circle of a 35mm full frame, certainly they can produce one nowadays to cover half that (Full Frame) in a mirrorless system. The X-pan was a rangefinder camera, but how that differs from a mirrorless I have absolutely no idea - I just looked through the viewfinder and pressed the tit. Magic happened and I got paid.....!