Alex Pankratov :
May 29, 2017
The bulk of the space in AppData\Local\Bvckup2 is used to store block hashes for files that were copied using delta copying method.
Here’s a detailed description of what it is and how it works - https://bvckup2.com/support/forum/topic/739
Long story short – if you don’t see yourself needing any of the delta copying benefits, you can disable it in Backup Settings > Copying section (change it to Copy in Full). You can get to Backup Settings by double-clicking on the backup entry in main window. After that you can also delete “/delta” folders in %LocalAppData%\Bvckup2\engine\backup-00xx directories to reclaim the space.