Hello Maccman,
When you delete a file on a hard drive, it's dumped into the Recycle Bin until you empty the Recycle Bin to permanently delete them.
When you permanently delete a file even from in the Recycle Bin, it's actually only marked as being deleted, and doesn't actually get deleted until it is overwritten by another file when it get's saved to the same sector on the hard drive. That's how a lot of the file recovery programs are able to recover deleted files. If they haven't been overwritten yet, they may be able to get marked as no longer deleted to be restored.
If you permanently deleted these files, and you have not regained the space they used on the hard drive in "Computer", then you could use the tutorial below to fix a corrupted Recycle Bin to regain the space from the deleted files.
Recycle Bin Corrupted - Cannot Delete File or Folder
Hope this helps,

Shawn