How can I remove unused data in garageband?

I recorded my band practice. This means I ended up importing 13 tracks, 2 hours each, into GarageBand. This creates a GarageBand project which is about 9GB big. I only want to keep about three minutes of this huge project. When I cut off the pieces I don’t want the file is still 9GB. It should only be a few hundred MB. How do I delete the unused data?