Originally posted by ZaphodB And three words, along the same theme as the bodies...
Not much difference.
I look at my APS-C dSLR with APS-C lens and I look at the same dSLR with a "full-frame" equivalent lens. Either way it's not pocketable. Either way it doesn't fit under a jacket. Either way it's something that takes up a significant amount of space in a bag, and requires the decision to go shooting rather than being something I can carry around all day everyday. Because it's a chunky dSLR with a sticky-outy lens, and being APS-C or 135 format doesn't really enter into it
Basically the "full-frame" body would be a bit bigger and heavier, and the "full-frame" lens a bit bigger and heavier. A bit. Not like the difference between the 35mm format and the 6x7 format. So from my point of view it's very likely that the quality advantages of the larger format will outweigh the weight/size advantages of the smaller one.
So tell me why the MF didn't overtake the 35mm film format?
Medium format can be surprisingly small, I have a 6x10 camera that is approximately 2x smaller than my K10 + grip.
You left out the most important side of it: price of a given lens for a given FoV.
One could argue for example that the DA*50-135 is 1/2 the bulk and cost than a 70-200f2.8 would be or compare a 50mmf1.4 to a 85f1.4...
Another thing is that the quality advantage we see in FF cameras might vanish as technology progresses.
It could go two ways: the APS-C could improve (we'll see about that in the next few month) or the FF quality could "decrease" (as in "cram" 22Mpix+ in a FF sensor and compare to a 10-12MP APS-C sensor: the pixel size is about the same so what you get is more resolution, everything else being the same, but how much more resolution can your optics resolve and how much more resolution do 99% of the photographers need?)
At one point, you'll reach a resolution limit (in close relation with the lens design) where the sole advantage with FF over APS-C will be to have the choice between a few more pixels or a slight increase in high ISO quality, both of which will be utterly unimportant for 99% of photographers because differences will be visible on 4x3m size prints when inspected with a magnifier, but you'll remain with the larger/more expensive FF lenses for a given field of view.
What will drive the market won't be size or weight, heck it won't even be IQ when differences will be so minute you'll need to go 100% pixel peeping to see them, it will be price and I don't see how FF is ever going to be cheaper than APS-C.