iCloud & iCloud Photos · Online-only files and Optimize Mac Storage
Why does Finder show a large size for an iCloud file that's online-only?
Short answer: Finder shows a file's full size in iCloud even when only a placeholder is on your Mac. Online-only files use almost no disk space, which is why Storage Studio measures the space files actually use on disk.
Finder shows the file's full size — how big it is in iCloud — even when only a placeholder is on your Mac. The space it actually takes on your disk is close to nothing until you download it.
That's why Storage Studio measures files by the room they actually use on your disk. In Storage Profile's Biggest space-users, online-only cloud files never appear as space hogs, and the space totals reflect what's genuinely on your SSD.
To check a single file, look for the cloud icon next to its name in Finder: if it's there, the file isn't taking up local space.