Okay, so it could be a bug, but it is possible that iCloud is secretly storing more than one version of the file in some cases? (After all, Apple does similar things with other media files.) The example given at the end is interesting: > So iCloud says the video is 128MB, I download it and the video is actually 48MB, and my free storage increases by ~170MB when I deleted it. Interesting! This suggests that iCloud isn…
So when you’re syncing photos to iCloud, it’s not just the individual files that get synced but it’s the “Photos Library” managed container of the Photos app.
If you add individual files directly in Finder or the Files app then their size matches exactly both in iCloud and on the local file system.