People who sit down and intend to write 400 pages of anything have a cute name: mental patients. Smaller things grow larger. Some of the greatest works were only 200 (or less) or so pages. Take your time and write what has to be written.
@Annihilation, you might try an exercise along these lines: Take a novel you've read and remember pretty well, but have not picked up in a few years. Remember a scene you enjoyed. Without opening the book, rewrite that scene in your own words, as if you were the author of the original novel. Chances are, your version will feel really short, given what you've already said about the trouble you have writing things that have a certain heft. When you're finished, compare your version to the one in the original novel. That should make plain to you what that writer did that you did not do, and should help you understand how the pros out there write good-sized novels. I hope this helps!
If you get down to brass tacks, it happens all the time. "The Hunger Games" was a 99k novel. I know it's not entirely accurate but using the rough 250 words per page and a 400 page novel is 100k words. That's well within the 80-120k range that most publishing houses want new authors' books to be.
Try reading books like Malazan Book of the Fallen by Erikson. "Simple" - far from it actually - characters and yes the action can be slow at times but it definitely doesn't lack. His typical books are 700+ pages. There one of the few more-modern books I literally couldn't put down - things like Harry Potter, Divergent, I can't pick up they're so simple. Really there's no secret or gimmick. It differs between everyone. The only way to write a good book that isn't going to drive the reader up the wall, is to love what you are writing. I know this first hand with drabbling in fiction in between my non-fictions [basically what I toy with as a pastime]. When I am really engaged in what I am writing, I can easily write 5,000 to 7,000 words of fiction a day [I could reach 100,000 in a month at that rate]. When I'm not - like now, hit a gigantic block in my non-fiction which has really put a damper on writing - I can go months without putting a word to paper. If you don't love what you're writing, you're going to be dragging your feet and the book isn't going to be worth the effort.
OurJud: You need to send what you have to other people and ask them for ideas regarding expanding the story. Also think about it yourself... and try to expand it in a meaninful way, rather like in a blow fish way. By the other hand, I like keeping short stories (and short story notes) at my reach, and usually play with those ideas whenever I'm working on a bigger project. So far I've found it useful. "Here I can use this!" Is the usual finding.
Reader's Digest used to publish "Condensed Novels"...I got one years ago with The Citadel by A.J.Cronin. It had been butchered to the point that you could see where great swathes of literature had been reduced to "Over the next three months, there were five more deaths".
