I will say that I do not believe this is a marketing ploy by Canon.
Clearly based on the limits of physical lens size, a mirrorless "full frame" system would use such large lenses that it would largely defeat the benefits of what is supposed to be a "compact" system.
Leica's awesome and expensive form factor - which provides full frame in a smallish body - works only because the lenses are small (because they are not autofocus and because of the short distance between the sensor and the rear element of the lens.) That photo posted earlier in the thread is a perfectly fair apples to apples FF vs FF 35/1.4 comparison.
In theory canon could easily recreate the leica system and of course it would be less expensive. However, they do not see enough demand for a system that, in order to be small and full frame, that would require the use of manual lenses (i suppose they could do something like contax g did with in-body autofocus motors - but that in-body autofocus approach seems to have enough downside that it never has had much traction elsewhere)
So throw out the possibility of leica or contax g type system (in order to keep modern autofocus,) and even cost no object, you then back into determining what is the best sensor to use based on the desired system size (including lens size.)
APS-C seems to offer as high ISO performance as the best 36x24 and medium format systems. So unless you have very specific depth of field requirements, or you are printing something gigantic, even with cost no object again - you would want your compact system camera to use APS-C. You create a new lens mount to provide a size advantage over comparable dslr lenses that require a shorter flange distance.
Comparing APS-C to g1x or nikon cx (1") or m43 - i just don't see those lenses and systems being small enough to justify the performance hit you do take by going smaller sensor. If I am going to start compromising to that degree i am probably willing to have a collapsible lens.
Suddenly, canon's offering makes perfect sense:
1" / micro43 / g1x size high quality compact (although perhaps best represented by the Sony RX100 at the very moment)
ef-m (bring on an optional model with some pro features like an evf, perhaps some external nobs for the people who want that all of course - i'm perfectly happy to omit them and save size and money. Fuji's x pro system is awesome but has some other downsides (for me body cost and autofocus speed) - and of course we canon fans feel security about the ability to attach our ef glass to the system - as for me i plan on having the 22/2 and the 85/1.8 in my eos m bag. ) most importantly, bring on the ef-m glass - a nice fast, prime 35/1.4 like the fuji.
ef-s (i could see this system dying off some day as the cost of full frame dslr continues to move downward - while a ef-s is a nice, compact size people will gravitate to the ef-m if the small size is the important factor and torward the ef if body cost becomes less a factor.)
ef (eventually all of their dslrs will be full frame - certainly their lens offering reflects this As the younger generation of non-viewfinder users grows - i could see a mirrorless ef for the benefit of primarily cost and perhaps some size and weight benefit. )
Perhaps a niche mount which is full frame and completely ef compatible would be created someday for landscape photographers?
