i 'finally and fully' let go of my FF dslr years ago, actually for apsc. what i didnt give up was my ability to compare weights and measures!
fuji apsc 35/1.4 (=52/2.0 for FL/DOF purposes): filter size 52mm, L 2.5", W 6.6oz
sony FF 55/1.8: filter size 49mm!, L 2.5"!, W 10oz.
again, only apsc benefit here is 1/2 stop light gathering which to many is offset by the FF 1/2 stop bokeh advantage.
fuji apsc 56/1.2 (=84/1.8): filter 62mm, L 2.75", W 14oz
nikon G 85/1.8, considered one of sharpest 85s ever: filter 67mm, L 2.9", W 12.3oz!
is there something factual or 'Laws of Physics-wise' i'm missing?
ok, lets do camera weights and measures:
X Pro 1 at 16mps: 5.5"x3.2"x1.7" at 16oz
Sony A7 at 24mps: 5.0"x3.7"x1.9" at 14.6oz!
do these results also run counter to the 'immutable' LoP's?
when i gave up my dslr for apsc there actually was a size advantage because i was trading my 5d and 17-40L lens for an X100. with the advent of mirrorless interchangeable lens FF cameras, and with zeiss devoting some expertise to compact lens production, that major size advantage just isnt there. its still there relative to m4/3, but its no longer that same striking difference, at least with fuji products.
Again, the general neighborhood of 50mm is where fast lenses can be built without much weight or size penalty. And your 85mm comparison with the Fuji 56 pretty much proves the point - the Fuji is a full stop faster than the Nikon and roughly the same size and weight. To go to the Nikon at f1.4 you get a lot bigger and the Canon at f1.2 bigger still. We obviously don't know what Sony will be able to do at that focal length yet.
If Sony can come out with wider and longer lenses that repeat the rough parity that they get to at 50-55mm, then we'll have more to talk about. But based on the 35, which is very small but also a full stop
slower than the RX1 lens and lower end (and also fairly small) Nikon and Canon 35 f2.0 lenses, I'm dubious. The f1.4 Canikon models where you equalize light gathering and maintain the roughly one stop advantage from the FF sensors are MUCH larger and heavier - 2-3 times the weight of the cheaper slower lenses. At 35mm there are f1.8 alternatives that more or less split the difference in terms of speed and size. The Fuji 23 f1.4 is basically in line with the f2 Canikon versions, a little smaller than the Canon and a little larger than the Nikon. Again, for the same (or very nearly the same) size and weight, you're losing a stop with full frame lenses, which is the rough advantage of the full frame sensors.
Doing a quick perusal of 20, 24, 28, and 85mm focal lengths (and Fuji equivalents) the same pattern holds. Canon's budget lenses tend to be slightly larger than Nikons, but not all that different and Fuji's equivalents are pretty much in the same size weight categories but a full stop faster. Canon and Nikon (and often Sigma) provide lenses that match the speed of the Fuji lenses for the full frame bodies, but they're significantly larger and, at least in the case of Canon and Nikon, vastly more expensive (Sigma not so much).
So I think the general point remains valid. For most focal lengths (other than the basically neutral lengths around 50mm), you can get Canikon lenses that are very similar size and weight to Fuji lenses at equivalent focal lengths at one stop slower, giving back most or all of the roughly one-stop advantage you get with the full frame sensor. To match the Fuji's speed with full frame and maintaining that one stop advantage, you're getting a lot bigger and heavier and, generally, more expensive. In some cases, there are manual focus alternatives that are smaller and lighter (although still generally quite expensive), and some people will be satisfied or very happy with those. But I'm talking about building a modern SYSTEM, which for most consumers needs to be built around AF lenses in almost every case.
In terms of bodies, I fully agree that full frame mirrorless bodies can probably be built about the same size as Fuji's largest APS body (the X-Pro), but Fuji, Sony, and Samsung all have APS bodies considerably smaller than the X-Pro. But clearly full frame mirrorless bodies can be built at very competitive sizes and weights.
The question still comes down to lenses. And I'm still far from convinced... Shocking, I know.
-Ray