It's similar in the Witcher 2, 3 and Skyrim. The saves start small and they grow to 10MB as you progress in the game. I guess there is a lot of world state and items to hold in an RPG. Count a few hundred saves, manual or autosave and that's a lot of space.

In the witcher 1 there is also an uncompressed BMP thumbnail with each save that is about a megabyte. It's PNG compressed in the witcher 2. They learned from their mistake.