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Well if you take a film body and want to keep it the exact same size but digital then the space that leaves you is what the spools take up and the pressure plate behind the film plane, that isn't enough for a battery, sensor, and all the associated electronics. I couldn't find a comparison of the RD1's thickness compared to a film rangefinder but it's thicker than the digital Leicas and they're thicker than their film counterparts.
Also an A7C is only a tiny bit thicker at the lens mount, but a fair bit less in width and height (A regular A7 is almost identical in height to the RD1, but thicker because of the viewfinder extending out the back). So if they rotated the battery and made the body a bit wider it could be comparable. Of course it would then be less ergonomic and getting rid of the grip makes no difference when your lens sticks out further.
