Originally posted by Medium FormatPro One cannot compare much of anything digital to the image sensor size on any medium format camera; there is no comparison.
A full frame 35mm camera has 2.32x as much sensor area as the K-5.
The 645D has 1.68x as much sensor area as a full-frame 35mm camera.
As you can see the improvement from going to 645D from full-frame is much smaller than going from the APS-C to full-frame.
I'd prefer a APS-C with a great sensor and a good lens to a full-frame with a cruddy sensor (say, from 2005) and a mediocre lens. I don't think I'm the only one, so although the 645D has it's advantages, to an even greater extent the comparison is going to rely on actual implementation of lens and sensor rather than sensor size itself.