In an interview with Red Dot Forum at Photokina, Leica head of professional products Stephen Schultz described how the all-new L Mount was built specifically for full frame, unlike the Sony E Mount.
Leica created the L Mount and partnered with Panasonic and Sigma to create a broader system. Though Leica initially called this mount the TL mount with the release of the crop body Leica TL in 2014, Schultz explained to David Farkas of Red Dot Forum that mount was always designed with full frame capabilities in mind:
The L mount was developed in order to have all kind of autofocus optics for full frame with sufficient diameter and on the other side, in order to realize to compact lenses for APS as well. It was not developed for APS.
Leica (with the crop body TL) and Sony (with the NEX-3 and NEX-5) both dripped their feet in the autofocus mirrorless digital camera market with a crop body. But Schultz said he believes Sony's development of the E-mount for the NEX line was without the eventual full frame camera in mind. "That’s a big difference with the L mount versus the Sony E mount. They started the mount with the NEX, which was APS. But, we assume that Sony wasn’t focused on full frame at that time due to the mount diameter. The Leica L-mount is much bigger, which gives us more flexibility," he said.
Schultz is absolutely right about the diameter. Sony's E mount has more in common with the Fuji X mount and Canon M mount, both designed for crop sensors. As you can see from this table, the E mount is closer in size to the dedicated crop mirrorless lens mounts from Canon and Fuji.
Schultz' belief that the E mount by Sony was created to be a crop body line makes a lot of sense when you look at the effort Sony was putting into their Alpha-series cameras at the time and how the lens diameter size lines up with other crop-specific mirrorless bodies.
Sony has obviously been on the cutting edge of the full frame mirrorless camera movement. But will their dedication to a mount size that may have been created solely for crop sensors hinder them now that Canon, Nikon, Leica, Panasonic, and Sigma have thrown their hats in the ring? Only time will tell.