The IIIG doesn't seem THAT much bigger to me, but it is a bit taller. width and depth are the same. The bigger viewfinder is great though.
Helen, the differences are: Bigger viewfinder, with parallax corrected projected framelines for 50mm, and corner markings for 90mm (thats the extra window on the front of the camera) , no delay dial to fool with for flash sync, (syncs at 1/5oth, or 1/30th and slower), bigger shutter speed dial, film reminder on the BACK of the camera like on the M3, (not on the wind knob as on the IIIF), Selftimer on all IIIG's, and a different take up spool- you can use the spool from the M3 and M2, which is great since I have an M3 and extra spool anyway.
The V/F, R/F windows are slightly different in size and location than on earlier screwmounts, so accessories like the nooky and such won't work, or the earlier contrast filters for the r/f windows.
I like mine, and even though a wee bit bigger than the IIIF's and earlier, it's still plenty pocketable w/ a 5cm elmar collapsed on the front. 🙂
-Brian