DSLRs are big (well compared to what exactly?) because when DSLRs came into market the idea of the camera manufacturers were to put the available technology inside the body (or a similar body) of the existing film cameras - the form factor remained the same. For canon that form factor probably drew on T90; for Nikon it was probably something like the body of Nikon F401 or F4. At that time there was a technological limitation on the miniaturization of the digital components. And photographers have grown accustomed with the form factor so much that there have never really been any revolutionary change in that form (What can Lytro lightfield form factor do is yet to be seen). In future the size is bound to go down a bit... with further miniaturization of the digital and electronic components. For example with the new AF technology of 70D I can foresee that the AF sensor at the base of the camera (the are located at the base of the camera right?) will surely be eliminated - so will the secondary mirror. As SL1 has shown the sensor mount mechanism can also be made smaller - in near future that will go to full frame as well, but making full frame cameras smaller may not be in the primary goal list of camera manufactureres.
OP is forgetting that the DSLRs need space for several motherboards, chips and much more wiring than the film/analogue cameras. Roger cicala of Lens Rentals stripped down the 5D III long back,.... it is worthwhile to have a look at what it has insidehttp://www.lensrentals.com/blog/2012/04/5d-iii-strip-tease