The Sony FE, Nikon Z and Canon R mounts all have flange distances shorter than the Leica M mount, yet Leica has the compact Summilux 35m f/1.4 design whereas the above's comparably speced lenses are huge and heavy monstrosities, surely this cannot be just to accommodate AF ?
Non-retrofocus, close to the "thin lens equation", strong wide angle designs (where the light comes in wide angle and goes out wide angle) are known to be tricky and unreliable in their results when used with many contemporary digital sensors, as opposed to using these with chemical film.
Digital sensors can exhibit artifacts like color shifts or loss of resolution in the corners when exposed with light rays coming in at a severe angle, often due to color filter matrices and on-sensor filters offering a different optical path than expected - for example, a color filter "pixel" (being a tiny bit of colored transparent material) hit almost sideways could end up bleeding some light right into the sensor-facing side of the next (differently coloured) filter element, literally undermining the color filtering effect there. If that doesn't happen (eg because whatever is next to the filter pixel acts more like a black baffle), that light might still not be fully registered by the sensor, resulting in vignetting.
Also, SLR lenses may appear shorter because they can outsource some of the bulk to a high focal flange distance body - the same lens on a DSLM adapter that restores the right flange distance will, again, become comparatively large.
Addon: IIRC Leitz actually had to do something special to their sensor filters for the digital M models exactly because of these issues...
As you can see, the f/1.4 version (in the rear) is taller, but not girthier, and it's still not all that large. It's about the size of an 85/1.8.
The reason more modern 35/1.4s are bigger is because they're designed for digital sensors, which require more correction for chromatic aberration than do film lenses. (Film is sensitive to different colours in a stack, primary colour atop the other, whereas digital separates this sensitivity laterally, one colour beside the other, and then assembles this data.) This requires more careful colour correction to avoid colour fringing in the image. This has led to greater use of low-dispersion and extra-low-dispersion glass in optical designs. In the film days, I don't recall ever seeing a wide angle lens that used ED glass, but you see such designs commonly now in the digital era. You also see more complex designs using larger number of elements to correct chromatic aberration. Some of these designs require larger elements.
To give you a sense of the size difference, while my Rokinon 35/1.4, a modern fast 35, takes 77mm filters but that 35mm Nikkor in the photo takes a 52mm, the same as Nikon's slowest 50mm lenses for 35mm and DX format.
Lens designs - there's many ways to design lenses of the same focal length, each with their own advantages and disadvantages. The Sony 35mm f/1.4 is a distagon design, where as I think the Leica is a planar.
Speculation - this has to do with patents - but then the Summilux is hardly new.
The US parent for the Summilux 35mm f/1.4 is https://patents.google.com/patent/US5161060A/en
And it appears to have expired for one publication but not for another ?
The Japanese patent is from Nov. 1998 - so should be expiring two months from now !