I give Olympus a LOT of well-deserved credit for the smallest and lightest pro bodies around (and make no mistake, the E-M1 and E-M5 II ARE pro bodies), for a bunch of great lenses (mixed in with quite a few mediocre ones, but anything they designate PRO is superb, as are some of the primes), and for one of the two sets of 'best designed cameras and lenses around" - the other one is Fuji. If cost, lens selection and image quality were out of the picture, photographers would argue between Olympus and Fuji endlessly as to what's more pleasant to shoot with, but those two would probably take 80% of the votes between them! Olympus adds "Hand of God" image stabilization to this - there's very little stabilized quite as well as an upper-end Olympus (although some newer lens-based systems come close, and some of the long telephotos may even equal it) - nothing else will stabilize a 50 year old Leica M lens like that, though!
One really interesting way to look at the size and weight advantage - four of capital's six systems offer a 24-70 or 24-80 equivalent f2.8 pro-quality zoom. All four are excellent lenses (well, no real tests on the Sony G-master yet, but preliminary results are very encouraging). There is no Canon mirrorless lens in that range (no high-end Canon mirrorless lenses exist at all), and the Sony Zeiss 16-70 (APS-C) is a stop slower than the others and not up to the same standard.
Olympus E-M5 mk II (14.4 ozs) or E-M1 (17 ozs) with Olympus 12-40 f2.8 PRO (14 ozs). Even with the E-M1, the total weight is under 2 lbs, and the lens takes a 62mm filter!
Fuji X-Pro 2 (17 ozs) and Fujinon 16-55 f2.8 (23 ozs). 2.5 lbs, 77 mm, unstabilized (neither body nor lens) - note that Fuji makes a very nice stabilized 18-55 f2.8-4 that is significantly lighter - not QUITE in the same optical quality range as the others, but fairly close). Choosing the Fujinon 18-55 f2.8-4, a compromise in aperture and weather sealing gets the Fuji into the same weight range as the Olympus.
Sony A7II (20 ozs)and Sony G-Master 24-70 f2.8 (31 ozs). 3.25 lbs, 82mm filter size.
Nikon D810 (31 ozs) and Nikkor 24-70 f2.8 VR (37 ozs). 4.25 lbs, 82mm filter size.
Add multiple lenses and the weight discrepancy gets larger between full frame and smaller formats - an A7II body weighs less than 1.5 times as much as an E-M1, but most of the lenses are more than twice as heavy as their Micro 4/3 equivalent (with Fuji somewhere in the middle). The advantage of Sony FE over FF DSLRs also declines - the lenses are similar in size and weight, and there are numerous focal lengths where Canon or Nikon have significantly lighter options (often with some other disadvantage). The half pound of body weight difference disappears as you add lenses.
Unfortunately, image quality runs more or less counter to compactness. If someone developed a Micro 43 sensor with the noise and dynamic range characteristics of a full-frame sensor, it would be child's play to build a full-frame sensor that was effectively FOUR of them, giving twice the linear resolution with the same noise and DR characteristics.
Right now, the sweet spot in sensor design seems to be the newest generation of Sony 24 MP APS-C sensors (Fuji X-Pro 2 and Sony a6300 - no DSLRs yet, but I suspect we'll see some this year) and the 42 MP full-frame sensor in the A7rII, which has a similar pixel pitch and also uses copper wiring and other newer manufacturing methods (the 42 MP sensor is also BSI, mainly to deal with lens compatibility, rather than improve IQ). These sensors seem to have a generational improvement in per-pixel characteristics over the earlier generation 24 MP sensors and the 36 MP full-frame sensor (although the 36 MP sensor uses superior resolution to come in ahead of the best 24 MP APS-C sensors in overall image quality). A Micro 43 sensor of similar pixel pitch would only be in the 10-12 MP range, which the market doesn't seem to want, so all Micro 43 sensors use smaller pixel pitch (with a resulting decline in dynamic range and increase in noise) to achieve 16-20 MP. When technology permits a 16-20 MP Micro 43 sensor that doesn't compromise noise or DR, it'll also be possible to make a ~36 MP APS-C sensor and a 64-80 MP full-frame sensor with the same technology.
At least to my eye, maximum print sizes run as follows (the bigger the print, the farther you view it from, so it scales a little differently than you might expect). I'm a landscape photographer, so other disciplines may also find this scaling different:
16 MP Micro 43: 12x18 on high detail subjects, 16x20 on most subjects, as large as 20x30 for a certain kind of highly atmospheric image.
16 MP Fuji: 16x24 on high detail subjects, 24x36 on some,but not all images.
Really good 24 MP (24 MP full frame, almost certainly the new 24 MP APS-C): 24x36 on most subjects
36 MP (I've never used the 42 MP A7rII, which I'm sure goes larger still): Significantly above 24x36 - how big is your printer?!!
Another important consideration is lens selection:
Truly complete systems (anything is available, including exotic telephotos out to 800 mm and multiple tilt-shift lenses): only Canon and Nikon full-frame - Canon and Nikon APS-C can use the full-frame lenses, so the whole range is out there at the cost of needlessly heavy lenses and some odd fields of view, both in primes and in zooms starting and stopping in unexpected places (a 24-70 has no wide angle capability on APS-C). These also have the widest selection of rental options, including big-city camera shops.
Essentially complete systems (everything EXCEPT a few exotic lenses): Fuji and Micro 43 (especially with a stabilized Olympus body). Both have most options you might want, with multiple choices at many focal lengths - neither has the oddest lenses available for the two big systems (tilt/shift, fast extreme telephoto, manufacturer fisheye (both have Samyang/Rokinon fisheyes, though)). Micro 43 has an advantage in affordable lenses, while Fuji has an advantage at the high end - either one is quite comprehensive. Significant rental availability, almost entirely online.
Adequate systems (wide selection, but depends on adapted lenses or lenses built for a different format): Canon and Nikon APS-C (using FF lenses), Leica M (very comprehensive within rangefinder limits), Sony FE (especially the A7rII with its full functionality with adapted lenses). Sony could really help their cause here with Sony-branded adapters. The brand-new Sigma Canon-mount adapter is a great start - there is finally an adapter that can be bought in camera stores, rather than primarily through eBay. Does anyone know if that adapter maintains the weathersealing? If you're willing to mess with adapters, the A7rII is the only camera that will mix and match Canon and Nikon glass at full performance...
Limited Systems (check and make sure the lenses you want are available and meet your quality standards): Pentax (both FF and APS-C), Nikon 1, Sony APS-C, Sony FE bodies that perform poorly with adapted lenses.
Dangerously Limited Systems (likely to be discontinued, limited selection and/or limited availability): Samsung NX, Canon Mirrorless (may expand with new bodies), Sigma SA (actually a surprising number of lenses "available", but no distribution - most are special orders even at B+H), Leica mounts other than M
