I always write a lean first draft, at least of novel-length stuff ... Novel drafts for me tend to hit about 80K and then the full novel is normally 110-115K.
This is an excellent and highly recommended strategy. David Baboulene recommends a 40k word first attempt, for instance, although preferred lengths will obviously vary. The chief advantage is twofold: you get a good solid rehearsal of your idea to shake the bugs out, and you also get a finished piece of work in the form of a novella or short story. The latter means you don't come away empty-handed in the event that you decide not to proceed with the full length novel.
This is probably a good way to get around the Douglas Adams syndrome I mentioned, above. Write the single good paragraph first, then write the four pages for the novel, later!
I've found I sometimes nearly almost use this technique by accident - I've yet to do it on purpose. I get halfway through a full length novel, then have a forehead-slapping epiphany about the plot that necessitates a complete rewrite of the first three chapters and multiple scene modifications and additions, throughout. If I'd done a 40k word first draft, instead, at least I'd have the satisfaction of finishing something!
But then, would I lose the motivation to write the full length book? Hmm...
Maybe fantasy is just meant to be long!
There are, in fact, unwritten conventions that a great many publishers still generally abide by with regard to the expected word count of different genres. Despite the rise of flash fiction, most commissioning editors will still regard anything less than 80k as a novella (and not accept a novella of less than 70k). Science Fiction and contemporary thrillers are expected to finish at around the 120k word mark, with an upper limit of 150k. Meanwhile, Fantasy epics are only just getting warmed up at 150, and can comfortably exceed 200k words without breaking a sweat.
We are, of course, talking traditional print media and to all rules there are exceptions. However, many of these conventions are so deeply engrained that they are unlikely to change quickly, and many have a firm basis in fact (I can't think of very many extremely long SciFi novels, for example. A quick glance across my shelves reveals that the vast majority are slim 120-150k paperbacks).