Recently, when opening a picture on my desktop with Windows Photo Viewer, it would eat up all the free disk space (~8 GB) on my Windows drive (C:) in a matter of seconds. As soon as I closed the viewer, the disk space was freed once again.
I tracked this down to the viewer creating a huge number of large temporary files in the
C:\Users\<username>\AppData\Local\Temp folder. Why this was done was not obvious, as I was just viewing a ~200 KB screenshot.