DSLRs are limited in size to their lens registration distance (distance from the lens flange to the sensor plane). There's no way of getting around that. You can remove the mirror, but you'd still have that fixed lens registration distance. As a result, DSLRs will always be much larger and thicker than these new mirrorless camera systems which are designed with much shorter lens registration distances.DSLRs could be a lot smaller than they are, and a few f/4 pancake lenses could make the package really small.Speaking of "small" -- I think the whole 4/3rds thing is about cameras for midgets anyway (nothing wrong with midgets, just saying...).
The design is frankly...stupid. Too bulky to fit in a pocket like a P/S, requires substantial investment in new lenses and accessories (if you already own another DSLR system it's redundant for the sake of "small"), and with the size comes awkward handling. A small DSLR makes better sense overall. I think the whole 4/3 thing is like the chihuahua dog craze -- "oh, it's a camera? It's so cuuute....!"
Here is an m4/3 camera with lens mounted, fitting inside a Canon Rebel DSLR body, a DSLR that is already considered small :
As you can see, a mirrorless camera system designed to have a shorter lens registration distance is much smaller than a DSLR that is limited in size by a much larger/deeper lens registration distance. And keep in mind that Canon's EOS SLR system has one of the shortest lens registration distances in the industry! Canon's EOS lens registartion distance is 44mm. Pentax K is 45.46mm. Nikon F is 46.5mm. As you can see, even with a pancake lens on a Rebel body, you're still talking about a camera that is much larger than an m4/3 camera! So, no, the reality is that today's DSLR systems really can't be "a lot smaller than they are."