You may have found this already: Why still thumbnails after clearing the cache?
Basically, Apple automatically generates these thumbnails in the Files app. They’re not in the original size. To be honest, I don’t know what kind of “clean up” strategy they have.
A lot of users actually like to have their thumbnails when clearing the cache (so that only the cached file, which is significantly larger, is deleted). Maybe we could add a feature request, which would allow the cleanup of thumbnails.