Help, need to make file smaller

I need to submit an assignment, and the file won't go any smaller.

It's a combination of audio and midi tracks. There are two 3:50 audio tracks that I made with a dozen takes, I flattened and merged them. There are a couple of shorter tracks with very few takes. Also flattened and merged.

The rest of the project is midi tracks.

I did the Project Management > Cleanup thing. That brought it down to 750mb.

I have tried completely deleting the audio tracks from the project, then running cleanup again, but the file size doesn't go down?! I think there are some hidden wav files somewhere that I can't delete? How to delete them?