I'm just messing around with different world spaces, and happened to notice that after adding Skyrim.esm as a master, TES5Edit saves the file, but adds 700kb to the size.

I thought at first that I'd made a mistake, so I tried again. Got a fresh esp straight from Tesannwyn, but this time, I only changed the Author from Default to Tamb0. Saved the esp. Again, 700kb added to the file size.

Can anyone explain what's happening?

Why did you add Skyrim.esm as a master? It's already a master isn't it?

When you create a new worldspace esp, you have to add Skyrim.esm as it's master, unless you're making a standalone world. (Game without Tamriel)
If you don't add Skyrim as the master, you can't use any of the assets from the game.

Sorry, I thought it was automatic. You should ask the guys of TES5Edit about that:

http://skyrim.nexusmods.com/mods/25859