Another crazy thing that I’ve seen in the past - in Photos on Mac, if you have a video and do Set as Poster Frame (or something like that to set the thumbnail),

Another crazy thing that I’ve seen in the past - in Photos on Mac, if you have a video and do Set as Poster Frame (or something like that to set the thumbnail), that duplicates the whole video. So if you have a 20GB video in Photos and you just want change the thumbnail, it’s gonna cost you another 20GB disk space