Note: this is not recommended. Make sure you have already had the data copied to the tapes, or other Disks, since you mentioned it's an auxiliary backup.
Navigate to the storage policy, and right click the disk copy you want to manage. Select View -> Jobs. Untick Specify Time Range, if you want to look at all jobs on the disk.
This gives you the details of all jobs. Size of Application is the data size on the backed up data size on the server. Size of Backup is the compressed data on the disks. Size on the Media is the compressed + deduped data on the disk if you have deduplication enabled.
Make sure the jobs have already been copied to tapes before you delete them, otherwise you will not be able to recover it.