Have you ever started a project that turned out bigger than you first planned?
Hmm.. Perhaps "outgrow" is the wrong word.
"Adapt" or "evolve" is better.
A concept I originally conceived as a comic book script will suddenly make much more sense as a short story. Or a novella will suddenly become a much better fit for a publisher reworked into a comic book miniseries.
That's what usually happens to me, anyway. Your mileage may vary.