My problem is that I don't think small. When I have an idea, it tends to be grand at minimum, and chances are it will become epic. I can't help but think about character backstories, world-building, interpersonal relationships, etc. Which isn't a bad thing, exactly, it's just that most of it is going to end up extraneous in a short story. I need to know these things to make the characters dynamic and the world believable, but short stories just don't give the same ability to delve deep into backstories and the world as novels.
What I tend to do is think "okay, well, I'll write the short story but know that I can always come back into the world later for a novel." Then I have editors tell me my stories feel like they're part of larger works, not really stories at all. Because they are, in essence.
I still haven't solved this problem. The best I've come up with is to just decide what this story is for this particular piece and keep it contained.
I'll let y'all know if it ever starts to work.