I was having a chat with one of the devs yesterday (on lemmy, so I guess you could find it in my post history). They realised that thumbnails are not resized at all. Thumbnails are generated when content is federated, and stored in full resolution. So even if no one looks at it, it's stored. And the thumbnail cache never expires.

Currently the image cache in total is 137GB (based on last night's backup), and has images from June when the instance started.

The image storing component pictrs has no ability to resize images, so looks like resizing isn't coming any time soon (it would be good to have to help thumbnails load faster). However, the next major update, lemmy 0.19 and pictrs 0.5, will have the ability to set a time limit on the cache (or even disable it completely).

If we aren't even resizing the thumbnails then perhaps having the cache disabled would be appropriate, though I had planned to set it to a few days or a week. In either case, there should be a dramatic drop in the size of the cache once that update is out.

A smart person may have waited until that was available before triggering the seeder, but I just saw the tab that's been open for weeks and decided to just get it done while I had the chance.