The book may not have been that long, but when actually put into cinematic form (plus with the addition of the White Council) it made sense for it to be longer than just one movie.

After all, we only made it halfway through the book by the end of this movie! And yet it moved pretty swiftly from one action to another, not many "just walking along" shots or anything like that. And what with all of Mirkwood, the elves, Laketown, dancing around Smaug, fighting Smaug, the politics between Thorin and Thranduil, and the battle of the Five Armies, there's still a lot to go! (Not to mention the White Council driving out the Necromancer.)
All said and done, I'm very excited for it!