The Sonnar on the rx1 looks deceptively small...but when you look at the lens once its pull off the body, you can see that its longer that it appears. The rear element extends into the rx1 body by quite a bit and sits right up against the sensor. Couldn't really use that same optical design on FE mount since the flange distance would be different.
I think the FE 35 f2.8 was Sony's attempt to mimic the Sonnar in the RX1, but obviously some compromises needed to be made.
I'm interested in seeing what Sigma has in store with their FE mount 35, but they're M.O. with the Art line isn't small glass. They sort of disregard size considerations altogether and throw massive corrective elements into the mix to get the best optical quality they can.

Carlo super informative, thanks. I'd bet a lot of the lens of the RX10iv goes into the body as well. Hey, learned something.
I'm sticking with the 35/2.8 and the SigART35 when I've been eating my Cheeries on a regular basis 









