There's more to The Hobbit than you might expect just from a page count.
The thing to remember about LotR is that they abbreviated the bejeezus out of those movies. Pick up Fellowship some time for comparison. They breeze over the long-expected party. They skip Farmer Cotton and Frodo's house in Buckland. Tom Bombadil disappears (yeah, a mercy, I know), which means that the Barrow Wights have to go. Hundreds of pages of story get nary a frame of film. Party, a quick chase, a heavily abbreviated Green Dragon, Weathertop, and poof! Elrond. All of Book One is zoomed through. For all that people complained about the way they dragged out the end of Return of the King, they completely cut the Scouring of the Shire, which could probably make a movie to itself.
The Hobbit is a much more economical book. It's aimed at children, not adults, so it has to take a breezier, less wordy take on things. It compresses a lot of events into a short book, and none of them are as easily cut.
Look at it this way: there are nineteen chapters in The Hobbit. Each of them tells a whole story, an incident with a beginning, middle, and end. If we allocate fifteen minutes to each chapter, that's nearly five hours. And some chapters are clearly going to merit more screen time than that, given Jackson's proclivities: he had to chop The Two Towers to shreds to make more time for beating up orcs. Can you imagine the Battle of Five Armies being dismissed in screen time proportional to the small number of pages that it actually gets?
There's clearly too much material for a single film. What would you trim. Beorn? The Hall of the Elven King? The Spiders? The Wargs? The Eagles? Each of these elements, to have a proper weight, is going to require a fair bit of screen time to work. Hell, he's almost certainly even going to have to trim Riddles in the Dark; eight riddles is probably too much.
And that's before we expand it with material that Jackson clearly intends to include: The Quest for Erebor, the council in Fellowship of the Ring, and the appendices from Return of the King. These are actually really important if it's going to be Peter Jackson making the movie. The Hobbit, as a book unto itself, isn't really exactly a prequel to The Lord of the Rings. It is another story in Tolkien's legendarium, but told with a distinctly different authorial voice and with a very different point of view. And an unreliable narrator at that: the Gandalf in LotR would probably be miffed to hear how Bilbo described him. The two stories contain so many inconsistencies that they can only be resolved by treating Bilbo as an unreliable narrator. Including a deliberate lie about his finding of the Ring, "corrected" in later editions to make the connection plausible.
Tolkien wrote a fair bit of material to try to merge the stories together. If Jackson wants to make these movies fit in with his existing works, he's going to have to use that material. That material is short in terms of pages, being told in the intensely terse style that he used practically everywhere except LotR and The Hobbit. It covers hundreds of years and introduces a whole lot of new story.
We already know some of what we're going to see: much more about the White Council, Radagast the Brown, the Battle of Dol Guldur, the Necromancer, as well as a lot more back story about the Dwarves. Oh, and some kind of frame story involving Frodo. I strongly suspect, reading between the lines of what we've been told, that Smaug croaks by the end of the second film, leaving the entire third film to talk about the Battle of Five Armies and the ends of the Dwarves.
The timing is still a bit unclear, and I am worried that it's going to feel just a little stretched out; I'm not crazy about Jackson's extended battle scenes. It's certainly not The Hobbit as you would conceive it to pick it up just by itself. But it could be three excellent movies. I might have been happier with the version that compressed them into two, forcing Jackson to sacrifice some moments that he really wanted to see that I could live without. But I think that comparing the number of pages in The Hobbit to three volumes of LotR is misleading: there's a huge amount of story there.
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