How much storage is taken up by the photos in iCloud and how much on the Mac?
Optimise Storage cannot remove the photos completely. You need at least the storage for the optimised versions, for previews, thumbnails, faces. Only the originals can be removed to save storage. On my Mac the size of the optimised library never dropped below 25 % of the storage needed for the originals in iCloud.
And the optimisation is fully automatic. Photos will tray to keep the recently used images in the full resolution, as long as you still have some free storage on your Mac. Only when you need to free storage for other apps, more photos will get optimised.
If you want to free as much storage as possible right away, try this user tip:
How to force Photos for Mac to Optimise the Storage Immediately
You can get everything quickly optimised by starting over with a new, empty library and a fresh download from iCloud. If you are sure, that all photos and videos are already in iCloud, try this:
- Simply make a backup copy of your Photos Library on an external volume, just in case something should go wrong.
- Then delete your Photos Library.photoslibrary from your Pictures folder.
- Now create a new, empty Photos Library. (see: Create additional libraries)
- If the Photos app on your Mac is open, choose Photos > Quit Photos.
- Hold down the Option key and double-click the Photos icon in the Applications folder (or click the Photos icon in the Dock) to open Photos.
- In the Choose Library dialogue, click Create New.
- Type a library name and choose a location to store the library.
- Click OK.
- In the Photos app on your Mac, choose Photos > Preferences, then click General.
- To make the current library the System Photo Library, click Use as System Photo Library.
- then make it the iCloud Photos Library in the Photos > Preferences > iCloud and enable immediately Optimize Storage.
Photos will now download your iCloud Photos to the new library and this new library will initially be very small and only have optimised versions.
There is one drawback: The projects will be missing, because iCloud id not storing the projects. And some named faces may be missing. Photos is storing all named faces in iCloud, but I frequently found that manually added faces circles have not been restored from iCloud. So it is essential to keep an archive copy of your Photos Library, before you let iCloud optimise it. You need a local copy anyway, because it is hard to backup an optimised iCloud Library. A local copy is easier to backup.