I suspect that the longer than average flange distance is the reason why I have a brace of tiny Samsung NX prime lenses that perform better across the frame (sharpness, vignetting) than their Micro 4/3 counterparts despite needing to project onto a larger sensor. They chose 25.5mm as the flange distance compared to <20mm for all the other mirrorless mounts. It would however seem that they are in the same boat as everyone else when it comes to zooms.I think that the other part of the reality of full frame mirrorless is that the rear element has to be pushed away from the sensor so that light hits the sensor as close to normal (perpendicular) as possible. My Pentax M 50mm F1.7 is tiny because besides being manual focus, the Pentax mirror box pushes the lens away from the sensor.
Leica's managed with micro mirrors and what not. I'm surprised that Sony has not done the same. Perhaps it's a cost issue.