Yeh it would be bigger than current m4/3, but mirrorless FF will still be a good amount smaller than DSLR FF, just look at Leica lenses, although I know they don't have AF though, but then sensor focusing will give you AF while still keeping the lens small. Let's see how big the FF nex is in a couple of weeks.
You keep claiming mirror less lenses can be made smaller than DSLR for the same senor size but I just fail to see that being true for most lenses. You can't just break the laws of physics nor ignore empirical evidence.
The size and weight of a lens of a function of the image circle, focal length and aperture. Flange distance doesn't directly impact lenses except for lenses with a focal length shorter than the flange distance which need to be retro-focus designs with a little added complexity. The flange distance on Canon and Nikon DSLRs is 44mm & 46.5mm, which is partly why 50mm lenses can be made so small. Mirrrorless cameras have a flange distance at around 18mm, the Samsung is 25mm. The Leica M-mount is 28mm.
For lenses longer than the flange distance then there is absolutely no size advantage. You want a 100mm lens then the front element will be around 100mm from the sensor. In fact, if the flange is shorter then for longer lenses like you 85mm prime the lens will have to be made physically longer and hence heavier.
Even for lenses wider than the flange there isn't a huge saving for many lenses. Ultra wide angle lenses will always be complex designs.. Larger sensors do simplify some aspects of UWA lenses but the larger image circle largely negates that.
Then there is the evidence, just look at the Sony NEX lenses compared to the Alpha lenses for APS-C.
Sony E (NEX) 18-55mm: 62 x 60 mm, 194g (small win for NEX)
Sony A 18-55mm: 70 x 69 mm, 210g
Sony E (NEX) 50mm f/1.8: 62 x 62 mm, 202g Nex is heavier)
Sony A 50mm f/1.8 DT (FF lens): 70 x 45 mm, 170g
Sony E 18-200mm f/3.5-6.3: 76x99mm, 524g
Sony AF DT 18-200 F3.5-6.3 73x85.5mm, 405g (Massive win for the Alpha)
There is no physical reason why most lens design will be smaller for a mirror less camera, and in fact for most lenses they will have to be made a little bit longer to compensate for the shorter flange distance.
In fact one sees that with the 18-20mm, the NEX version has to be longer to maintain the same focal length.
A 35mm prime might be able to be made smaller for a mirrorless FF but that would be about it.
Looking at Leica lenses is just misleading because they don't auto-focus and are based on older simpler designs. If you look at the longer lenses then they have absolutely no weight saving at all: the 135mm is 450-500g for a measly f/4.0 (f/3.5 some models). In comparison the Nikon 135mm f/3.5 is 409g. Similar build quality, same lack of AF, basic physics dictating that both lenses are about the same size and weight despite the Leica having a shorter flange. There was even a Nikon 135mm f/2.8 made unitl 2005 that was 435g, so you can have a stop faster, at least double the weight of glass in the front element and the same or lighter weight lens.
Lets look at the 50mm f/1.4, Leica weighs in at 335-460g depending on metal (alu vs brass), The Nikon, 290g in plastic. The Leica is 54x53mm, the nikon 73x54mm. The extra width of the Nikon down to the autofocus motors.
OK,OK, plastic vs metal, Lets look at the Nikon 50mm f/1.4 AI-S in solid brass, no auto focus. 40mm long by 63mm wide and the weight... 248g. SO How has the lack of mirror and shorter flange helped the Leica, not at all really. Coincidentally the Nikon 50mm f/1.4 AIS is a nice example of what a simple lens design looks like, the total distance from sensor/film to front of lens is an exact 50.5mm. Funny how a 50mm lens needs to be 50mm or more from the sensor
And then there is the simple economics of the fact. Lets say Nikon dumps the mirror (something that will definitely happen sooner or later). Do they ditch their entire line of F-Mount lenses starting from nothing, or simply keep the flange distance the same and maintain their f-mount with full compatibility to all lenses. Thom Hogan suggest the D5 could be mirrorless, and it makes a sense a sports camera drops the clunky mirror. Do you honestly think Nikon would do anything other than maintain the same mount?
And even if they did shorten the flange distance to have a shallower camera and have to redesign the entire lens line up. Why do you think they wont take the current lenses and just extend the barrel, want an 85mm f/1.4 for Nikon short flange mount, simply add 15-20mm of plastic around the base of the 85mm G lens and off you go, no added cost, cheap redesign, use all the same old component.
And then look at the Canon 100D. It is truly tiny and light (400g) yet maintian the entire mirror and OVF. Ditc the mirror and you can save a chunk of space and knock another 70g or more format he camera and still maintain the same EF-S mount with the same flange distance.
Bottom line:
Mirrorless is the future but don't expect smaller lenses, or even small bodies at the pro line - ergonomics alone means that larger bodies are desirable for larger lenses. When Canon and Nikon shift to mirror-less they wont be changing their mount, especially Nikon. If you want small lenses get a camera with a smaller sensor that allow lenses with a smaller image circle.
