A feature available in iOS, Mac, and Android allows users to optimize local device storage by storing full-resolution photos in the cloud and downloaded on demand.
iOS
Before enabling Optimize Device Storage, please verify your data is synced by checking the Server Dashboard. See here: How do I know if my data is synced?
Open Day One > Settings > Sync > Storage and choose either Optimize Device Storage or Download All Media.
Please note that with the optimize device storage option enabled, full resolution media may not be included in the iCloud device backups. Only media that is stored locally.
After enabling "Optimize Device Storage", you can clear local media storage by tapping Clear Local Photo Storage. The data will still be stored on the server.
macOS
Open Day One > Preferences (in the file menu) > Sync > and choose either Optimize Device Storage or Download All Media.
Android
Open Day One > Settings > Sync > and choose to either turn on or off Optimize Device Storage. It is turned off by default.
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.