Is writing a short story very different than writing a novella or a novel? My own experience suggests the answer is no. Good writing is good writing, and the goal is to achieve quality no matter what you write. But surely, you say, there must be some differences when composing these different forms. Actually, in my opinion there is, but again, not in the writing.
The difference lies in the conception of the story you wish to relate. That is, when I begin, the length of the story provokes a different way of thinking about my story.
When I thought of writing Crispin, the Cross of Lead, I thought of it as a multi-volume story, and almost immediately conceptualized what the four volumes would be. (Well yes, the fourth volume has never been written, but I still know what the plot of that fourth volume is, and I even know the last moments of the tale. Someday…) When I wrote Sophia’s War, the overall arc of the story was broadly defined in my head, right from the first page. Even when I wrote City of Orphans, I had the overall complexity of the story in my head. No, by no means did I have it all sorted out—far from it—but I knew I wanted to write a novel.
However, when I compose a short story, and I have just been writing a few of them, my sense of the plot begins with knowing that the tale I intend to write will be short. That in turn informs my writing.
Yes, I could take the same idea and recalibrate it into something longer (that has happened) but most often, I choose not to.
I suspect it would be the same if you are a runner. If you intend to run a marathon, your thought process would be different than if you were going to run a fifty-yard dash. Note that in both cases, you are running, but you run in a different way. And when the “Start” gun is sounded you know what you have to do.