Originally posted by ElJamoquio That depends. The K-5 with a limited prime is quite small, but those who start from a 7-D and 17-55 would see a size reduction going to the 6-D and 24-105 f/4.
We can't really predict how much smaller the FF lens any APS-C lens would be on FF, or how much bigger (or smaller) the camera is going to be. In any case, though, everyone should realize FF isn't going to be bigger than APS-C for much longer - bigger cameras are not a requirement of FF, and most lenses between 24mm and 70mm (equivalent) are smaller on FF. Above 70mm you can crop on FF.
OK, now I'm really curious... how does a lens that has to cover a larger sensor end up being the same size or smaller? And how does a camera with a larger mirror and sensor end up being the same size or smaller than one a smaller sensor , mirror, shutter etc.
It would stand to reason that using the same technology in both, an APS-c lens will always be a few mm smaller, as will the camera body. Some things, like the pentaprism, the sensor and the lenses will always have to be bigger.
At the very best the lenses will be the same, since any lens used on an FF can also be used on an APS-c. Same lens same weight. The 15 and 21 however can not be used on an FF, and I've yet to see. You could easily argue that since you have to use a 600mm lens to be the equivalent to a 400 on APS-c, FF lenses are much heavier for the same FoV.
I really don't understand what you're thinking.