How not to write short scenarios

It's happened plenty of times. I have the glimmering of an idea, and sit down to dash off a quick scenario. Something short, simple and punchy. Suitable for a con game, perhaps.

Then something happens. I'm not sure what.

What emerges from the fog is a vast, creaking beast numbering a couple of hundred pages and weighing in at a quarter of a million words.

It happened with The Perishing of Sir Ashby Phipps, my gentle Victorian mystery.

It happened with The Wolf Who Cried Boy, my modern conspiracy cleanup crew adventure.

It happened with the "short, introductory adventure that touches on the distinctive features of Call of Cthulhu" I started, currently under the working title of Cried the Lady.

And the same thing with the Wolf Boy prequel I have in the works.

So seriously, how do people actually write short, one-shot adventures? Because if I could work that out, I might get a whole lot more things written.

I suspect this partly comes down to my own playstyle. I'm an inveterate busybody, always eager to poke at the setting and see what's there. Who maintains the machinery in the abandoned fortress? Wouldn't the chef know roughly how many people were at dinner? If the players rent a top-floor apartment across the street, what can they see going on in the grounds of the mansion? I naturally think about these things, and just as naturally put the answers into what I write.

And certainly I've been frustrated before by scenarios that don't consider what seems like a plausible course of action. That probably plays a part. What if I decide that actually, we aren't so different, you and I? Or call the police and report the obviously criminal actions of the highly-arrestible demographic, instead of mounting an illegal armed raid in search of evidence they're also doing something else that isn't illegal?

It's not that I want to stop writing long scenarios. I enjoy the process of assembling the whole thing, lovingly placing a minor NPC here, a handout there. But I'd like to have short-form writing in my toolkit as well, and as yet it's not something I feel at all skilled with.