Accepting a submission call for a story of limited length forces you to be your own editor. You have to discipline yourself in telling the story you want to tell in as few words as possible.
For me, writing has a purely instinctive stage. I start by just getting out what I want to get out and worry about whittling it down and cleaning it up later. I like to turn the characters loose and let the scenes take on a life of their own.
But, as you keep one eye on the word count, you realize you have to go back and decide what to sacrifice. How can you convey the same information in fewer words? Which information or character expression is unnecessary? What's the best way to streamline each scene effectively? And, which scenes are completely unnecessary? You start to feel like you're deciding who to push out of the lifeboat. But, then you remind yourself that you'll never improve as a writer unless you learn to cut the flab from your own work and let the story become an instrument to get the point across as effectively as possible.
You write for yourself, but learning to develop an editorial facility means you're writing for the reader as well. That means getting to the heart of it without sacrificing the soul. Hard sometimes, especially when you're having fun with a story. But, it's like dieting; learn to internalize a regimen and you'll be pleased with the result.