Originally posted by K David FF sensors with in-camera stabilization require larger mounts.
I doubt this is the case; after all, there were non-stabilized FF cameras with mounts smaller than Pentax K. And the Minolta mount can accommodate in-body stabilization.
Of course, you're absolutely right when saying there are many things on the digital camera which simply needs space. The SR system itself, all the electronics, the back LCD... lots of things were added, compared even to a modern AF film camera.