Disk Space in cPanel
Your cPanel keeps disk space usage for your entire account, including email accounts, databases and other applications. Disk space usage figures are cached for faster interactions. The numbers will refresh once every 4 hours.
If you import a database and can see all of the tables and data, then the data is saved and is using disk space. Your cPanel may still show 0 MB, but this will be updated within 24 hours.
If you purge many emails from the trash and can see the folder is empty, then the emails are gone and are no longer using disk space. Your cPanel may still show 20 MB being used, but this will be updated within 4 hours.
Disk Space in WHM
If you increase the total disk space for an account, and WHM shows the increase, then the account can use the new disk space immediately. The cPanel for the modified account may still show the old disk space limit, but this will update itself within 4 hours.