It is probably the new screensaver from MacOS sonoma. According to the review on ars technica they are quite large but use purgable storage. Purgable storage is a way to keep files on disk but as soon as something else requests the space the purgable files are deleted.
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[ 1.8 ms ] story [ 17.4 ms ] threadhttps://arstechnica.com/gadgets/2023/09/macos-14-sonoma-the-...