The "small (but thick)" artbook that comes with the Premium set is the same size as an Amaray case (so it fits in the chipboard case) but will have more pages. Those extra pages will mostly be devoted to the settei (model sheets). The "large (but thinner)" artbook has 8.5x11" pages, and focuses more on the color artwork, plus a selection of the more important settei.
Both artbook variants will have all of the new art created for the release (new color illustrations, the new manga, the dojinshi, the winners of the fan art contest, and so on) as well as a selection of the color art created for the original release of Gunsmith Cats.
One way to look at it is that the small artbook is a reference work, and the large artbook is more like a coffee-table book.