I occasionally hear writers of big thick multivolume series comment that they don’t know how to write a single-book story or, worse, a short story. But for me, short stories and short novels are easy (insofar as any writing is easy); I tend to think in 60K or less (often far, far less) sized story chunks, and to write on that scale. Actually, learning to write something even as long as a short novel took some work for me; I came into writing via short stories. So I look at thick multivolume series with a sort of admiration and awe, because while I enjoy reading a good one, and sinking deep into its world, the thought of writing one is pretty overwhelming.
So my question here is: for those of you who write long, how do you write long? What sort of things make a story expand outwards and further outwards, rather than within a couple hundred pages turning in again on itself?