A new feature introduced in version 2.5.9 for iOS allows users to optimize local device storage by storing full-resolution photos in the cloud and downloaded on demand. 

iOS

Open Day One > Settings > Sync > and choose either Optimize Device Storage or Download All Media

After enabling "Optimize Device Storage", you can clear local media storage by navigating to Day One > Settings > Sync > Storage Used > Clear Local Photo Storage.

macOS

Open Day One > Preferences (in the file menu) > Sync > and choose either Optimize Device Storage or Download All Media.

Photos are optimized when imported to Day One for Basic and Plus users for a more efficient storage and sync. Premium users can now store higher resolution photos. See here.

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