With CommVault Deduplication there are two major components:
-DDB Volume on the MediaAgent -> this stores the Primary and Secondary Database Table, that keeps track of where the actual data (sigantures) are written on the Disk Library (Storage Array or Cloud -> depends on your setup).
-Disk Library -> will then store the actual metadata reference and signature that build the job.
When you run your first Backup, these are all unique Blocks, and the references will be inserted in the DDB Volume to identify the location on your Storage, where it is written to.
Deduplication is the process of referencing existing blocks on the Storage Array to reduce data consumption.
So although you delete jobs (Deduplicated), the blocks written on the Storage Array, could be referenced for other Jobs.
However if the original Job that you ran was non-deduplicated, when you do delete that Job, it should free up all the space.
So in short when you do delete Deduplicated Jobs, the amount of space being freed will depend.
Advantages of having DDB Volume to keep track of the signature and the Disk Library actually storing the metadata and signature -> in the worst case scenario if the DDB Volume is corrupted, we are still able to restore data from the Disk Library, as additional metadata reference is also written to the Disk Library, which removes the need to interact with the DDB.