Sometimes you may face the issue with cPanel Disk space calculation between total disk space used and total E-mail folder size. For Example, the entire cPanel account showing you 100 MB while one of the Email accounts is showing the 350MB large size which is surprising.
In this case, please run the below command using SSH.
# /scripts/generate_maildirsize --confirm --allaccounts --verbose $USER
Note: replace the $USER with your actual cPanel user.
If the issue persists, log out the cPanel account and delete the email_accounts.yaml and email_accounts.cache files from the cPanel account's .cpanel directory. These files will be created automatically while you log in to the cPanel again.