Save storage space when syncing on Mac
You can set up syncing to have large music files converted to different bit rates or to have video files use standard definition format to save storage space.
See Intro to syncing your Mac and your devices.
Connect your device to your Mac.
You can connect your device using a USB or USB-C cable or using a Wi-Fi connection. See Sync content between your Mac and iPhone, iPad, or iPod touch over Wi-Fi.
In the Finder
on your Mac, select the device in the Finder sidebar.
If you connect your device to your Mac using a USB cable and don’t see the device in the Finder sidebar, see If your device doesn’t appear in the sidebar.
Click General in the button bar.
In the Options section, select the storage options you want:
Prefer standard definition videos: Allows your Mac to sync only smaller standard-definition videos. Larger high-definition videos take up more storage and your device may not be able to play them.
Sync only checked [songs] or [videos]: Syncs only the songs or videos that you selected in the Music or TV Show options in the button bar. This option only appears when you have “Songs list checkboxes” selected in General settings in Apple Music or “Checkboxes in library” selected in General settings in the Apple TV app.
Manually manage music, movies, and TV shows: Turns off the automatic syncing of music, movies, and TV shows. Doing this can help you save storage space on your device, because you can choose which music, movies, and TV shows are transferred.
Convert higher bit rate songs to: Allow room for more songs on your device by having your Mac convert high bit rate songs to lower bit rate songs that take up less storage space on your device.
Click Apply.
Before disconnecting your device from your Mac, click the Eject button 
in the Finder sidebar.