My K-x measures about 63mm from flange surface to the back of the rear LCD panel, whilst my MZ-30 measures only 48mm from front to back (protruding part excluded). Actually, I believe you can see the big difference from the photos above.
Recently, I am very interested in getting the Samsung NX10. It is just because I still found my K-x is just too big and heavy, especially when I am on a business trip (when I already have all my documents and my notebook/netbook computer). As the NX10 is equipped with an APS-C sized sensor but its size/weight is significantly reduced and is actually much comparable to my micro-4/3 Pana GF-1, which has just a much smaller sensor (50% less in light receiving area when compared to the APS-C).
I think Samsung has been absolutely right in concept for making a compact and really lightweight APS-C EVIL and system. Recently, I have test driven the NX10 for several times. Whilst I like many of its design and features it offers, one thing that I am still not convinced with it, or with any Pentax DSLRs using a Samsung sensor, is that its produced Image Quality is somehow lacking/lagging behind. Unnatural colour rendition is a problem, in particular for the skin tones, which is actually very important for portraits - one of my major daily photographic needs. And more so for the obvious higher noise of the Samsung sensors, which IMO is a real problem.
In fact, I did find that the ISO 400 of my GF-1 is more or less something similar to the ISO 800 of the NX10 which in fact could be looking even a bit worse than what I can get at ISO 1600 with my K-x. :-(
So, could the Pentax DSLRs using the old K-mount be made thinner? I suppose Yes, possibly. If a film SLR like my MZ-30 can be made at 48mm for its overall thickness, why not a Pentax DSLR? What Pentax needs to add in is the CMOS imager, with Shake Reduction module and the back colour LCD monitor. Would that 1.5 cm difference be just too much? Anyway, the SR module would be the most space-consuming part there, I believe. But however, do note that for film SLRs, there is also space required for the film pressure plate at the back cover.
Well, if Pentax can further minimise their DSLRs, for its size, weight and overall dimension of a lens+body combo would be like this..
..which can even be a full frame DSLR! Then, I am almost sure that if they can make it, they will gain much of the DSLR market share! Just imagine if a Full Frame (Pentax) DSLR just looks and feels like my above which is just at around 400g which is ready to use with Lithium battery (but not AAs, which is large, heavy and of low performance!) and SD card, then!?
Reference:-
Camera Mounts & Registers