I've read short stories that are as dense as a 19th century novel and novels that really are short stories filled with a lot of helium.
Once you've invested hundreds of hours in creating a coherent universe, your story's grown to around a half-million words and can't be written as anything less than a trilogy.
For me, writing a short story is much, much harder than writing a novel.
I'm not constrained by being a genre writer. Any story I can imagine, I can cast as a fantasy novel and probably get it published.
A good short-story writer has an instinct for sketching in just enough background to ground the specific story.