Why are DSLRs so big? Try putting a 70-200 f/2.8 on a Sony A7 and see how well it balances and handles
Large & heavy full frame lenses need a large body otherwise it is a royal pain to try and grip the camera body + balance the lens.
I actually far prefer the balance of the 400f5.6 on my Rebel T3 over the 5D2 with battery grip. The 5D2+grip weighs almost as much as the lens itself, meaning you pretty much have to hold the whole thing with two hands all the time, whereas with the Rebel it pivots on the Tripod foot and is much easier to swing around from carrying it by your side to lining up a shot. It feels a lot more nimble with a lightweight camera on the back.
I don't believe there's any reason a smaller body would be a compromise in control. Look at current Fuji models, they have more buttons and dials than any Canon. At any rate, I think it would be amazing if Canon would at least start developing light weight versions of their high end cameras. From what I've heard a lot of photographers tend to have pretty sore arms after a long day of shooting.
Errrrr on a tripod yeah, it won't matter especially with it mounted on the collar. Try handholding and it's a world of difference, you ever do an all day event or wedding shoot? The 70-200 F/2.8 is an all around great lens to use but man, back when I used a rebel with one it was a pain, halfway through the day my wrist was not working the way I wanted it to. Weight unbalanced towards the front, front-heaviness, is a lot more of a pain to deal with than getting a bigger heavier camera that actually balances out the whole setup. The bigger grip is comfortable to hold and the whole thing is easier to swing around. And really, ~ 800 grams is not bad for just the body when carrying a load of lenses and other gear, once you add a flash as well then shaving off several more grams on just a single body like with a mirrorless camera (lets say A7) doesn't do much.
Fujifim's cameras are that way because their lenses are also small and lightweight, and neither are they FF, which helps reduce the size and weight big time.
Aside from grip and balance though, DSLRs on their own aren't that big, even a 5D series, take the lens off and hey it can fit in a coat pocket
It's the big lenses, get a pancake lens like the 40mm F/2.8 and it's all compact enough.
The only reason I'd be excited about a mirrorless FF mount is for adapting all sorts of lenses to... lenses I don't have anyways though... small and lightweight bodies would need equally small and lightweight, but still fast and high quality lenses, and I'm not looking at Leica as they currently don't offer AF