Read this interesting bit in an interview with Helen Marshall:
One of the odd things I’ve found as a writer of the fantastic is that the longer the story, the more the story is forced into realism, even if it has an absurd or fantastic core. Long fiction is about tracing a series of consequences, and so it must be tied together by a believable reality. Short fiction…not so much. It doesn’t have to be sequential. It doesn’t have to be consequential. You can get away with so much more, and that makes it particularly good for horror stories. Horror, to me, is about confronting the fact that we live in a world that doesn’t actually make much sense.
Perhaps the reading brain can only handle total unreality in smaller doses.
I’m considering her statement in relation to an unpublished long story of mine called “Mountain.” I think that in the case of my story, what I was doing required a certain amount of realism and length, though it’s set in an unreal environment, and it ends in unreality. I have lately tended to write longer, but am drawn to the type of story she describes.