Originally posted by RGlasel The thing about digital cameras is everyone wants a unicorn, built to their individual specs. To realize any size benefit from mirrorless cameras requires smaller lenses, which means a smaller sensor. M43 is the largest format where mirrorless has a clear advantage. Put equivalent lenses on a downsized APS-C DSLR like the SL-1 or K-S1 and an A77 and there is no size advantage when taking pictures. If building a smaller camera was such an advantage, it would be FF cameras facing extinction, not the true compacts with 1" and smaller sensors. Cameras without mirrors have been around longer than cameras with mirrors, electronic viewfinders have been around since the fifties on video cameras. No matter where you turn, photographic equipment is mature technology.
Sure, the sensor impacts the size of the lens but it doesn't mean that all the potential size benefit of a mirrorless camera is negated in all circumstances. The Pentax 43mm, if natively mountable on a mirrorless camera, would result in a more compact kit than when mounted on a K-1. For some lens-body combinations the body is the bigger partner, and shrinking it does make a difference. Mirrorless undoubtably has the potential to be smaller over a conventional DSLR. However, sure, some lens types can't be miniaturised effectively - tele lenses spring to mind. But even in these cases most mirrorless cameras will be lighter and have less volume than an equivalent DSLR. Ergonomics and balance is another issue of course!
I also think that depth is a big part of perceived size. The shorter registration distance allows for a mirrorless camera to be thinner, and I think this matters to perceptions of size, even if overall volume is similar.