That's really quite interesting... In the "About This Mac" Storage pane, does it show as "Other" or Purgable?
Do you use Time Machine? TM keeps local backups as well, but it's supposed to automatically shrink when you need the storage for other things
You can try and wipe the local TM backups by writing this in Terminal
"sudo tmutil disablelocal"
This disables local backups and deletes the backups
"sudo tmutil enablelocal"
Re-enables local backups but fresh, so none of the old backups are stored locally anymore, and your pervious backups are only on your actual TM disk