Using up disk space

When I work on a project, I have gotten into the habit of creating a duplicate of the project prior to beginning edits, so if I have to go back I can grab the duplicate. As I progress I have duplicate 1, duplicate 2, etc. Eventually I get a notice that I am our of hard drive space, so I eliminate the duplicates. I am somewhat confused, however, since I thought projects were only markers to the media and therefore used up little disk space. Could there be something else using up the disk space? I thought once I had imported my media, that there would be very little disk space used until my project was finalized, and I always make sure I have at least 100 or so gigs available after my media is imported. It is clear to me that I do not yet understand the basis upon which FCPX is working and any enlightenment would be appreciated as to how I can avoid that reocurring message when working on my projects "You are almost out of disk space" requiring me to stop and clear disk space.