Story length is a tricky thing. There's often a certain length that a writer naturally falls into — the way some folks are sprinters, and others long-distance runners. It's as though my mind can only come up with so much plot; then I tie it off and move on. But I never set out with the ambition to write just a short story, or a novel. I start with a beginning, and whatever entanglements emerge from there do so without thought to how long they'll take to resolve.
I think the key to writing a novel is the presence of sub-plots. It's really hard to ride a single idea through those 60,000 words. There need to be interruptions along the way so that you're not just rehashing a single problem. Naturally, it takes a lot more work to weave in subplots that are relevant but distinct. Unfortunately, romance for me lends itself to focus: one man, one woman, and how they come to be together.
Or maybe I'm just lazy.