I don't understand some of the fascination with a smaller, lighter body. The whole advantage of the DSLR is that the lenses are interchangeable. If you aren't going to carry additional lenses, why even have a interchangeable lens system. And, if you are going to carry additional lenses, the weight and bulk of those lenses make the bulk of the actual camera kind of irrelevant.
In fact, one of the of the advantages of the DSLR over the mirrorless cameras is with large lenses. A small mirrorless might have some advantage with a lens up to about 135mm equivalent. But, once you get beyond that, does it really matter how big the camera is? (BTW, I just looked at their website and it looks like Leica doesn't even sell a lens longer than 135 for their M system)
Same with the fascination with a full-frame mirrorless system. If the main selling point is size and convenience, much of that will be lost with the larger format.
I think you're exactly right. You can't have a small system unless you can keep the lenses small, and beyond a certain lens size ergonomics makes small camera bodies a nuisance. Until there's some technological revolution that lets FF lenses resemble M43 lenses in size and weight - as you point out, Leica don't have anything longer than 135mm, and how small is that lens anyway? - the new small mirrorless Sony FF bodies will only provide a size advantage for those interested in lenses of a rather limited focal length. I would love it if a bag of FF lenses could be as light as my bag of M43 lenses, but....