Your cPanel keeps disk space usage for your entire account, email accounts, databases, and other features. These amounts are cached for faster interactions. The numbers will refresh once every day.
So, no worries. 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 4 hours.
If you purge many emails from the trash and can see the folder is empty, then the emails are really gone and not using disk space. Your cPanel may still show 20 MB being used, but this will be updated within 4 hours.
WHM users, 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. Inside that cPanel may still show the old disk space limit, but this will update itself within 4 hours.