Comment 2 for bug 807476

You can get the size used by all snapshots by getting the size ("nautilus" or just "du -sh <path>" command) of the folder:
   <SNAPSHOTS PATH>/backintime

I see that you backup virtualbox drives (*.vdi). This file can be real big and change every time you start the virtual machine.
You should check this files sizes.

The problem is that BIT remove old snapshots only after it takes a new one (because it doesn't know how disk space it will use).
If you take big snapshots you should change the rule to "if free space is less than 50gb" (or more).

Regards,
Dan