I have the total, opposite question. I'll have an idea, and think: "Aha! I have a good short story here!" I then flesh it out, and add neat things to the idea, throw in some characters, a little history, and then, before I know it, I have 50,000 words.

But the one thing that made these stories grow uncontrollably was characters demanding that their stories be told, demanding that we get to know them, demanding (not asking permission, demanding) to wander off on their own sub-plots. I learned to eliminate characters mercilessly, nipping them in the bud before they got lives of their own. Some of them squealed--and might come back in longer versions of the same stories, better now for the focus the flash version defined.

I'm fascinated by this phenomenon. How characters develop their own lives, aspirations and personalities inside one's head is a mystery, one of the pleasures of writing fiction. But the implication of your question is correct: the growth has to be controlled somehow or the work gets out of hand, loses focus and becomes unreadable.

I think that editorial control--what's in the story, what's out--is one of the key aspects of good writing. For me it means asking, of each character, each sub-plot, "Does this move the main story forward? Is this the only way of adding something essential? Is it interesting and sustainable in its own right?"

These questions have to be asked especially of each character, no matter how minor. In one of my WIPs one character is quite vulnerable and may be eliminated because the answers to those questions are "yes", "possibly" and "no" and I think that answers other than unequivocal "Yes's" indicate areas for possible elimination. One can only do it with distance--not immediately after the first draft is finished when one feels passionately about all the characters.

(I think that a story can grow like this: The answers to those questions about a character are, "Yes, moves story forwards; yes, only way of adding something essential; no, not interesting and sustainable." Then, the story has to be expanded to give this essential character space to develop an essential role and a clear identity. Which I suppose is where novels come from.)