Verbatim from this KB - https://support.symantec.com/en_US/article.TECH154127.html
"Deduplication folder does not delete or truncate empty containers. The space seems consumed from Windows Explorer but it is available for use."
There are large container files created as the dedupe store grows. These are immediately created, and take up space. There may or may not be actual "data," within them. Think of them as an empty thick provisioned VM...
Over time, dedupe data gets put within the container file. That data can age out, get marked for deletion, and bits within the container files overwritten. But each container file will still consume the same amount of space, it's a fixed value.
Part of this design is done for performance reasons. If the container files were allowed to grow and shrink, the disk fragmentation would be insane. The other reason is how Symantec bolted on essentially what is an AIT VTL tape library, and how it works with media sets/tapes. There are compaction and maintenance tasks one can run, but they are painfully slow, and used more so for troubleshooting than they are for proactive maintenance.