Database size doesn't get updated in cPanel.

If the database inside the cPanel shows 0 mb, it is because the database cache file of the user is not updated. 

To fix this, login to server via SSH and edit the below file:

vi /var/cpanel/cpanel.config

check disk_usage_include_sqldbs=0 and change it to disk_usage_include_sqldbs=1, if the line noe found, simply add it to the bottom. 

You can do the same via WHM >> Main >> Server Configuration >> Tweak Settings >> SQL >>
Include databases in disk usage calculations >> Set that to On.

Then run the below command:
/scripts/update_db_cache