I've often heard it said that writers have a "natural length" - set a particular author loose on a story idea and their stories will tend to come out in a particular length range. This also applies to the idea of "natural novel writers" whose ideas like to expand into longer books, and "natural short story writers" whose ideas are pithy and can be concisely expressed. Some people find it really hard to cross over this borderline; others find it less difficult.
I believe in natural length. I began as a novel writer, and for the longest time I was convinced that short stories just weren't for me. Then, once I started writing them, I learned a lot of things about story structure that I could bring back and apply to my novels. However, it turns out I do have a "natural length" for short stories - it runs between 7500-12,000 words. This is called the "novelette" length. I never set out to write a novelette; I just have an idea, organize it and write it... and guess what? It's a novelette.
I think that the difference between novel and short story writers may have something to do with structure. It's hard to say, but I do notice that I find it easier to take a large idea and render it in a small form than to take a small idea and render it in large form. When I take a large idea and shrink it, I remove everything that doesn't tie directly back into the main conflict thread. When I take a small idea and expand it, I often find everything I add in feels like fluff. Maybe it's just that I like the potent feeling in every sentence of the short form.
Another factor, though, might be what I call the fractal structure of novels. People talk a lot about the "hook" and about "arcs" and about the "climax" etc. The more I write, the more I realize that this same kind of structure happens at multiple levels. Each chapter needs to have its own hook, its own raising of stakes, its own climax. To some extent, even each smaller scene has this same pattern on a smaller scale. It's no wonder that novel chapters can so often be used as short stories!
This may be one reason why it's tricky to move from short to long. A string of short stories does not make a novel, because they'll have structure on the lower level, but they may not have structure on the larger level. That large-scale, across-the-novel trajectory may be difficult to identify and work into the progression of the shorter pieces. I don't mean to say that it can't be done, but I know I would find it tricky if I had created a sequence of stories and hadn't come into it already thinking about how to create the larger-scale pattern.
Have any of you had experience switching from long to short, or from short to long? What was it like for you? Have you taken a particular story and tried to change its format from novel to short or vice versa? I'd be interested to hear what your experience was like.