Backup Statistics

Is it possible to get the total size the backup files themselves are taking up on the server other than using the OS to read the file sizes? The stats on the web interface say I have 400GB but the actual backup files take up 2.5TB. I assume the 400GB is the size of the incremental backup and not the actual physical files themselves? Or am I looking at something incorrectly?

I am using a linux install if that makes a difference.