I don't believe we have any automatic cleanup routines built-in to SmarterMail for this, but you could clean these up manually to recover the space you need. The mailing list's previously sent messages list exists as a folder on the server's disk, and within it you'll find GRP files containing that mail data just like a user account has. So, you can clean these up and it'll return that missing space! I'd recommend a process like this one: 

1. Stop the SmarterMail system service and monitor the Task Manager until it drops from the Details tab. We don't want to make any disk level changes while SmarterMail is in memory. 

2. Navigate to the Mailing List directory on the disk (variation of C:\SmarterMail\Domains\domain-name.com\Lists\MailingList-test\Previously Sent Messages) make a backup of the files you want to remove, then delete those GRP files and the mailbox.cfg file.

3. Start the SmarterMail system service back up and confirm you've recovered that missing space. 

I did a quick test on this here in my test server and did not see any adverse affects on the mailing list itself, but I do recommend backing up those files before you purge them. Let me know how that works out for you!