Certainly a handy term to have. I've always thought of underwriting and overwriting as terms to use in the context of drafting, that they're different styles of building a story, maybe not so much applicable to outliners/architects/plotters, moreso a thing of discovery writers/gardeners/pantsers. I fall in the latter category, and tend to alternate underwriting and overwriting; it depends how fast the ideas are coming in as I chart a course. If I underwrite I'll leave it sparse and fatten it up later, if I overwrite I'll come back with a hedge trimmer. Have to say I'm more on the overwriting side of things, at least historically. These days I often hit it decently close to the middle.
That being said, I fully embrace underwritten and overwritten as terms to use for finished products. Just hadn't occurred to me to use them as such.
Every story has its natural length, certainly, and that's a hard spot to find. If I look at my "finished" novels they're probably a bit overwritten across the board, but I'm the sort of writer who doesn't mind my stuff being a bit clunky and bloated if that means more flavor and texture and lore. If you look at it from a commercial viability standpoint then my work probably has too much stuffing in places. I leave the majority of my ideas in, so whatever else I might be doing wrong I don't underwrite. I also never start big projects with a set word count in mind, since I don't plan them ahead of time whatsoever. It's something I might wanna do for future work.
I think the best thing you can do if you feel your stuff ends up too short is just let it sit for a while, then write another draft or two. Things have a way of solving themselves when you're not looking.
