Sometimes, Mailman archives consumes lot of disk space. There is no any option from cPanel to delete these archives. User may enable or disable archiving via the mailman interface. Once archiving is disabled then the archive may be removed manually via SSH access. We need root access the server for performing this operation.

Steps to delete archives:

1. Login to the server as root via SSH.

2. Go to the directory "/usr/local/cpanel/3rdparty/mailman/archives/private" using the command given below.

cd /usr/local/cpanel/3rdparty/mailman/archives/private

2. In this folder you can see the list names. Now go to the folder (list's directory) which consumes disk space. Let's consider example_lists.test.org consumes more disk space. 

cd example_lists.test.org

In this folder, you can see many files in the format "year-month.txt.gz" (Example:2010-August.txt.gz), "year-month" (Example: 2010-May) and a folder called "attachments". Now you can remove these folders. If you wish you can take a backup of the folder before removing them from the server.
After that, update mailman cache
/scripts/update_mailman_cache