Reply to Music management issue - iTunes (Mac)

Notable info: I have my itunes set to copy a tune and organize it in it's folder when i open it from my download folder. I then regularly delete the contents of the download folder to save space.

Whenever i delete a song, i select to move the original file to the trash (which i then empty).

Today I found that the itunes music folder has about 57 gigs of data, but itunes reads 36 gigs.

When I open the folder a massive list of folders with artist names show up, in each is the song, etc.

How do i go about opening all of the songs int here in itunes? If i do, will it only open sons that are not already there?

Is there an easier way of knowing what that extra 20 gigs is?

Thanks!