Writers, ever sit down to write a short story...?

and find that you've written a novel instead. I'm currently writing what use to be a short story. It's ten pages which is fine except, I'm only halfway through and those ten pages are more of a detailed outline with dialogue. Ever have this problem? Any ideas or suggestions for cutting it down? Should I even cut it down? Or just let it grow into its own thing even if that means writing a trilogy? I'm 50% sure everything is necessary although I could just be suffering from AAS (attached author syndrome).