Mac storage basics · Photos, music and video
Why does adding songs to the Music app use double the space?
Short answer: Music → Settings → Files has Copy files to Music Media folder when adding to library ticked, so songs are copied and originals remain. Untick it or delete the originals.
If Copy files to Music Media folder when adding to library is ticked in Music → Settings → Files, Music copies every song you add into its own folder, leaving the original where it was — so you have two copies.
Either untick it before adding (Music will then play songs from where they are), or delete the originals once they're safely in the Music Media folder.