Depending on what you are doing within FCP X and the type of footage used 100 gig can disappear pretty fast.
HD footage grabs around 40 gig an hour plus any render files that is created along the way.
My guess is render files are accumulating.
Have a look in the top level of the drive where the Project is stored to check out the current size.
Final Cut Projects>Project Name>Render files.
Just to add to Al's comments about accumulating renders…when you duplicate a project, a dialogue window pops up with several options. The "lightest" way to operate is to choose the top option and leave the "Include Render Files" box at the lower left on the window unchecked.