Originally posted by Adam Because the sensor is smaller, they just make the whole mirror box and focusing screen small as well. It's more cost-effective, as the bigger the mirror, the harder it is to make a high-framerate shutter.
The istD had a full-frame mirror box, but after that, Pentax reduced its size down to APS-c likely for the reasons above. I'm sure there's an ff on the horizon, though- and if you really want a big VF, try the 645d!
IMHO this has nothing to do with the size of mirror and mirror box. Put as large mirror as you want, however you still need to show in viewfinder only the same area of image which will fall onto sensor. Therefore you can't put larger focussing screen either. You can increase the size of the image in viewfinder itself, however then viewfinder will become even darker...
I've never used *ist D, so I can't say anything from my experience, however dpreview says that *ist D has 0.95x magnification prism viewfinder with 95% frame coverage. I.e. it's the same size as viewfinder in *ist DS, K10D and K20D, all of which have APS-C sized mirror and mirror box.