The M-mount, with a 58mm throat diameter, is unfortunately not that much smaller than the EF mount. The MFT mount, on the other hand, has a 38mm throat diameter and the NX system a 42mm throat diameter. If Canon wants to imitate these systems then, yes, they might introduce a smaller lens mount.
I wonder actually whether the reverse might be true for a FF mirrorless system. Look at this shot of the internals of the RX1 lens...
That's one heck of a big rear element(looks too large to fit in a regular E-mount), I'm no optics expert but I wonder whether its there to help deal with the issue of extreme light angles hitting the sensor? certainly the RX1 lens seems to be more compact for its specs than any of the FE lenses(the 35mm f/2.8 is about the same size but a stop slower with worse macro ability, the new 35mm 1.4 far larger).
So perhaps whilst it may technically be possible to fit FF coverage into an EF-M lens its actually a better option to go with something that has a larger mount diameter to allow for a big correcting rear element and so smaller lenses?
m43 might have a smaller mount size but relative to the sensor size its actually a lot larger than the the Sony FE system.