“The Secret Life of Walter Mitty” (1939) is a short story by American humorist James Thurber. The story has been adapted several times for film, most recently in 2013. I haven’t seen the films but it’s interesting someone financed feature length movies out of a story so short — “Walter Mitty” is 2,512 words.
“Brokeback Mountain” is a short story adapted far more successfully for film (though not according to Annie Proulx, because they butchered the main message). “Brokeback Mountain” is a capacious, novelistic short story, and and gives the director far more to work with, coming in at 9,135 words.
My theory is that sometimes short stories (and picture books) are simply too short to fill a feature length film. Scriptwriters must artificially bulk them out. Unfortunately, it’s hard to make money out of short films, which they probably should be.
A reviewer at The New Yorker is rather more optimistic about this than I am:
The great thing about adapting a short story rather than a novel is that it demands expansion rather than reduction—it’s a springboard, not a blueprint.