I’ve seen many times that iOS devices will retain as much as gigabytes of old items—especially in the Messages app—that it doesn’t purge when it’s no longer needed locally or has been deleted. The only way to restore this missing storage is perform a backup and restore, which wipes the phone. This dumps allocated but unneeded files, though it’s a time sink. (Apple’s guide will walk you through it.)
You can also connect an iOS device to a Mac and use iPhoto, Photos for OS X, or even Image Capture to transfer images, then delete them after they’re moved over. You can also opt for third-party cloud storage services, like those offered by Amazon and Google, to back up images and videos and store reduced-size versions or previews on your phone.
Terence Nickolette asks:
Apple doesn’t provide 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 such point as whatever threshold is reached where OS X (and iOS with that feature enabled there) starts dumping any locally stored full-resolution original image files.
But you don’t have to manage this. Once you move the library over, as long as the files have been synced to iCloud Photo Library, they’ll be deleted from your local machine without your involvement. I expect a 45GB library could be as little as 2GB to 5GB after optimization.
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