How to delete unused file parts that are not used in the FCPX timeline?
As you already know you can't easily export just the used parts of your clips. But given FCPX uses XML there must be utilities to do that.

You can get something like clipexporter that will do it automatically.

However you can "share" your final project and then slice that up into shots -- I don't know why you want to do that slicing. AVcutty will slice AVI's automatically (windows OS only).

If disk space is a real problem and you don't mind losing your original footage, just share your final timeline.

Personally I buy an external HDD every two years (whatever $200 gets me) and archive all my personal projects to that. I then copy all the old projects from the old HDD to the new HDD.