Not that I am an expert, but I think there may be dangers in getting too curious when exploring some kinds of hidden files in the user's home folder when using a desktop GUI. In my case, it is with the Linux Mint (Mate) desktop.

The hidden folder is .cache/thumbnails/normal, along with 'fail'.
DO NOT casually open this with a mouse click, because it seems to go into some kind of recursive runaway size expansion creating thousands of what I think might be "thumbnails of the thumbnails".

At first, I was checking it out because 'normal' had seemed to have grown rather large, but all I could see was the scroll-bar forever shrinking until I realised something was wrong. When I managed to halt the process, I used a terminal to delete everything in it.

I tried it again later with only a few thumbnails in there, with a similar result. To see the thumbnails, if you must, it might be better to first copy the 'normal' folder to some other name.

I do not have enough life left to explore which (Ubuntu?) desktop configurations might be prone to this. It just happened to me with Mint Gnome2 lookalike (Mate), so I thought I would mention it here.

Change your file manager to show directory contents with "advanced" or just "basic" information?

Creating thumbnails of the thumbnails - and then continue creating new thumbnails of the thumbnail-thumbnails and so on - sounds like a bug in MATE's file manager (caja?) or some GNOME2 libraries. You could search if this is already reported to the MATE people and let them know if not.

The usual directory for shared thumbnails is ~/.thumbnails. With KDE's dolphin nothing unusual happens when opening previews in that directory and its children.

This must be a new bug: Mate is based on Gnome 2 code and it doesn't happen in Gnome 2. The only problem there is that there doesn't seem to be any task to clean out thumbnails: after a few years, they really accumulate! I have a startup script that deletes thumbnails and flash cookies.