In Divinity, the save folders are a little large, but are also fairly compressible. You can free up a lot of space by zipping older saves. When saving, the game writes the files to a temporary folder first, in case there is a problem during the save (ie the original save should still be ok if you were over-writing one), so needs a bit more room for this. I don't recall the exact save procedure, but checked it out once with a program called Filemon.
In Beyond Divinity, the save sizes should be smaller.