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.
Open Day One > Settings > Sync > Storage and choose either Optimize Device Storage or Download All Media.
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.
Open Day One > Settings > Sync > and choose to either turn on or off Optimize Device Storage. It is turned off by default.
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.