#cd /usr/local/cpanel/3rdparty/mailman/archives/private
You can see lots of directories in this path.
For example, If your mailing list account name is example@workonlinux.wordpress.com
then the directory name will be
example_workonlinux.wordpress.com
#cd example_workonlinux.wordpress.com
In this directory, you can see many files like
2012-January.txt.gz(year-month.txt.gz)
2012-January/(year-month)
attachments/
You can remove these files and directories.
#rm -rf file/directory name
Also one more thing, SOmetimes it will show the previous file size when you try to access it from cPanel front end. It may be due to cache. It would take around one hour to show the correct mailing list size after deletion.