File too large

My song files are 1.5 GB and have 30 audio aif files each for keys, bass, etc. When I record and redo a track I click undo (command-z). How can I get files smaller?
Also when I open some songs a window comes on that some tracks are unmapped or missing. Why? Each song is in a folder. Is there a way to retrieve lost tracks that are mixed up and showing up on a different song?
Thanks for your help.


What Sample Rate and bit depth do you use? You can reduce the file size here.

When you open a project, you are informed some Audio files are missing. You can search them directly from that window.