The trouble is, when I start early I believe the fic I'm writing will be 5K, and budget my time accordingly. I don't discover its irrepressible need to be 20K until 5K of it has already been written.

Since I am capable of writing 5K fics (the second fic will be 5K!) it always seems to me that I could simply write a 5K fic on the first try for the next exchange. This apparently is not true, but it feels so illogical that I can never convince myself of it.