Hi Sander,
This is the expected behavior. Previews are stored as part of each history state for all image windows.
As you recall I requested an option to hide the preview windows without deleting them. Is that still on the list?
Of course. I am about to release 1.4.1. Let's see if I can add an option to ImageWindow - I don't promise anything though, since this involves a complete recompilation of the core application (on all platforms), and a new bug-fix version is urgent now.
Anyway, if not in 1.4.1, you'll see it on 1.4.x for sure.