some of Mac’s users found that photos has filled up their Mac’s memory and get confused because after trashing other images they still got a full phone, and doesn’t want to turn to iCloud Photo Library to help.
Sometimes iOS devices will save as much as gigabytes of old items—especially in the Messages app—that it doesn’t purge when it’s has been deleted. The only way to get back this missing storage is do a backup and restore, which wipes the phone.
Also you can connect an iOS device to a Mac and use iPhoto, Photos for OS X, or even Image Capture to transfer images, and delete them after the transfer processes are over. Or maybe using the third-party cloud storage services, like those offered by Amazonand Google, to back up images and videos and store reduced-size versions or previews on your phone.
When does optimization kick in with Photos?
Apple is not providing any public guidance about how optimization works, but moving the library won’t immediately provide any relief. A 45GB library will remain a 45GB library until it reached where OS X starts deleting any locally stored full-resolution original image files.
But don’t worry about this. Once you move the library as long as the files have been synced to iCloud Photo Library, they’ll be deleted from your local machine. Expecting a 45GB library could be as little as 2GB to 5GB after optimization.