As someone who has both the X10 and the GRD III I can say the following: The X10 is not much bigger than the GRD III - it's about the same length (maybe a mm or two longer); it's a bit taller (by about 1/2 a cm maybe) and it's "thicker" due to the lens and other components.
The X10 is not as pocketable but then again, it depends on how/where you carry your camera. The X10 in JPG kicks the butt out of the GRD III at ISO 1600 and above. You can turn the noise reduction down on the X10 so that the high ISO images don't appear "mushy" or "soft".
So far, between the two of them I would say the X10 is edging out the GRD III (or GRD IV for that matter) in the areas of image quality, High ISO, functionality and the fact that there is a zoom that's reasonably fast on the camera. Where the GRD will win is in ergonomics (I have yet to find a camera in this size that matches the GRD in this category), size, and "ease of use".
Cheers,
Dave