Maybe FF mirrorless size saving would be greater with a larger mount that can allow such big rear elements to avoid issues with light dropoff and boarder softness?
no MAYBE there. The BIGGER the freakin' hole in the body compared to sensor inside is, the EASIER it is to design
- optically better and/or
- smaller and lighter and/or
- less pricey
Sony/Zeiss FE lenses are so ridiculously expensive and huge, because that mount hole is simply too small relative to the FF sensor inside. Sony made a huge mistake using the APS-C sized E-mount hole as basis for their new FF mirrorless line. It is a totally unnecessary weak spot built-in from the very start and this achilles heel will hurt them ever more year after year, as competitors will come to market with better, cheaper and more compact lens ranges for FF sensored MILCs.
Canon EF-M lenses are so damn small, damn good and downright cheap, because the EF-M mount - like all current Canon mounts EF7EF-S/EF-M - has a really generous clearance and is very well-sized relative to the sensor inside [and to the chosen flange distance]. And no, EF-M cannot possibly handle an FF sensor in an uncompromised, meaningful and quality fashion. Luckily Canon has repeatedly also stated this and will not compromise on that one.
I wouldn't be surprised, to me the whole FE system looked like it was thrown together in a bit of a mad rush to be first to market and could potentially highlight why that isn't always a good idea. You look back to the AF revolution and the EF mount was actually quite late to the party but ended up dominating by providing better performance.
On other difference I could see with a Canon FF mirrorless system would be that unlike Sony it likely wouldn't be trying to replace FF DSLR's but rather aiming for specifically at the small camera market. Sony are IMHO clearly trying to use this system to makeup for their failure to build more market share with the FF A-mount, Canon aren't in that situation. I could see the end result being rather similar to the EOS M system, for the kind of market who buys a camera like that(generally in Japan) the lenses are very well targeted, especially in terms of not crippling kit options the way Sony tend to, I could see a similar kind of lineup for a FF system, half a dozen or so lenses would probably give most users what there after(normal/UWA zooms, f/2 primes plus maybe a short f/2.8 macro) anything else could be adapted from EF.