Having moved completely from the big Canon DSLR/lenses to Sony, I am firmly in the 'small camera system' camp. Given that I am also a small person with small hands, it helps to have a smaller unit.
I know many people have difficulty with the form factor of the Sony esp if they have big hands, but hey, the world is moving on, more and more Asians are buying these cameras and it is the time of the small folk now
Seriously though, I agree completely, technology should allow the manufacture of smaller, lighter long lenses (yeah, an oxymoron, that) - the Canon 400 DO MkII was pretty good, but did not win the popularity stakes. I am hearing good things about the Nikon PF lenses and I am sure Sony can come up with something similar. A zoom lens in the 200-600 range would be fantastic, does not have to be f4 either. I know Tamron makes one in a similar config, but my own experience with it was less than satisfactory. You want to use it maximum at the longest end and that's where it fell short.
I also agree that sometimes you want that separation from the subject and the background and nothing beats a wider aperture for that, especially if the animal is at a certain distance. Just cannot do it with an f5.6. So the need for a heavy long lens continues. I have come to the realization in life that good things cost money and all other things being equal, good images require good equipment.
I too would love to see the FF Sony bodies become even smaller and personally I don't have issues with big lenses on small bodies, as long as it balances well.
I love my Rx1r2 but of course it has a fixed prime lens.
I suspect the future A7x and A9x bodies will not be any bigger, rather the same size but may have more features built in, whether we want them or not.