Quote Originally Posted by Dougjgreen
The main reasons are - speed and power and viewfinder and sensor size. Because Full Frame DSLRs are built for professionals, it needs to deliver speed of operation. The fact that the sensor electronics are larger, and need to operate at high shooting rates, means that the power supply needs to be more robust to deliver lots of power in high current bursts that are more substantial than in a smaller sensor camera that shoots at slower frame rates. So the entire power supply portion of a full frame DSLR tends to be bigger than a smaller-sensor SLR. Similarly, a small sensor SLR has the same issues compared to a compact point and shoot. This is not pixel count, it's the physical size and number of transistors of the sensor electronics that drives this.

And finally, the viewfinder has to be larger, to cover the wider frame, without losing brightness. Because these viewfinders need to be high quality, they use a pentaprism, which is a big chunk of optical glass, the same as what's in a pro-level 35mm film SLR like a Canon F-1n or Nikon F5. Also, the mirror box and shutter mechanism must be larger to cover the larger sensor than in a smaller sensor DSLR.
Ok, that makes sense. I didn't consider the power pack and the physical scope of the electronics.

Yet I'm not clear on the difference with regard to the prism and shutter mechanism; wouldn't these be of equal size in a full frame DSLR to its film counterpart? Since I don't know the F-1n or F5 you'll have to humour me by explaining how these are different from a "regular" 35mm SLR.

@Anbesol - thanks for your summary about sensor size. I probably should have explained that I was making an example of how the technology has decreased the size of compact cameras, whereas with DLSRs, the reverse is true.

Ta,