The difference between the two is comparable to the difference between WotC's Gazetteer and Living Greyhawk Gazetteer. The mini-gaz is 32 pages, includes a fold-out color poster map, and offers a very basic overview of Greyhawk. The LGG is 192 pages and goes into much greater detail. Both are softcover, although the LGG is perfect-bound (squared-off spine), while the Gaz is staple-bound (like an adventure).
With the Scarred Lands books, the Gazetteer includes a fold-out color map and a brief overview; it's also staple bound and 48 pages long. SLCS: Ghelspad is a 256 page hardcover, and provides a lot more info on the setting.
From a DMs point of view, one nice thing about this approach is that you can give the smaller gazetteer to your players and keep the more detailed setting book to yourself -- that way they know a bit about the world (enough to aid in making a character), but don't have the full picture.