Maybe I'm just stupid, but I've never understood the obsession with word count among amateur webnovel writers-- i.e. people who have total and complete freedom to make a story as long or as short as they want. o_O Like seriously, I'm no closer to understanding this than I was when I first got to this forum. To me, word count as a concept stopped mattering when I got out of high school...
Anyway, to actually answer the title question: any adaptation of any written work is gonna have changes made to it so it can fit the new medium, whether that means adding things or subtracting things. Shorter works (or works that have been chopped up to make a series, as in the 'trilogy' trend) will simply have more things added-- this is totally normal and expected, and it happens all the time. I mean, just think about all the movies that have been made out of fairy tales, which can often be retold with just a handful of sentences. '_'
So there's really nothing to worry about...any sufficiently skilled and motivated writing+directing team will be able to make a decent movie out of any story, even if it's only 10 pages long, let alone 10 thousand words.