You can reduce the app's file storage by changing your Music Video Files settings.
Tap Manage Memory under Music Video Files to set the number of music video files you would like to save to your device.
If the number of music video files exceeds the cap you have set, the files will be deleted in order of oldest to newest. The necessary files will re-download when you play the song again.
You can find these settings from Menu > Options > System > Music Video Files.