I recently heard a reference to a famous Hemingway legend, about the old master of brevity writing a six-word short story to win a bet: "For sale. Baby shoes. Never worn." Apparently the truth of this story and Hemingway's authorship are up for debate--but never mind that.
These words strung together seem to have a powerful effect on most people encountering them (thus the legs this story has maintained), and for me it is a wonderful example of an emotional arc constituting a short story. There's lots of talk out there about emotional arcs, plot development, character growth, dramatic movement, etc., etc., and etc., making up what we commonly understand to be a story--and hey, all of that stuff is great. But for me, the only requisite thing is the emotional arc, whether we're talking about an epic novel (which may contain many such arcs) or a piece of short fiction.
Some people hold that a character or characters have to change in order for a story to have been achieved. I say that characters need not change, but that the real issue is how our understanding of them moves (or changes) from one emotional place to another. Like all somewhat mystical, highly subjective notions, it's best not to try to assign any tight metrics to this sort of thing. But think of it: "For sale. Baby shoes. Never worn." An enormous and wrenching implied emotional arc in a play on a classified advertisement. Talk about the tip of the iceberg.