After 4 days of having Nautilus open to that same folder on both machines with gnome-video-thumbnailer forcibly disabled, Nautilus is still only using 100MB on one machine and 88MB on the other. So lacking any other differentiating factor that I know of, I'm inclined to say the memory leak was caused by an interaction between nautilus and gnome-video-thumbnailer.
After 4 days of having Nautilus open to that same folder on both machines with gnome-video- thumbnailer forcibly disabled, Nautilus is still only using 100MB on one machine and 88MB on the other. So lacking any other differentiating factor that I know of, I'm inclined to say the memory leak was caused by an interaction between nautilus and gnome-video- thumbnailer.