Symptoms
Disk usage at Subscriptions > example.com > Websites & Domains > View More Statistic page shows incorrect data for additional domains and subdomains.
Cause
This is expected behavior: the disk space occupied by web content of subdomains and additional domains is included in the disk space usage calculated for the main domain.
The value of disk space usage calculated for a subdomain or additional domain includes only system files such as configurations and logs of the domain, plus content of mailboxes of this subdomain/additional domain.
Resolution
Currently, there is no such functionality in Plesk to see the statistics of a subdomain or additional domain via the Plesk user interface.
If you would like to see this feature in Plesk, please vote for it on Plesk UserVoice:
Note: To review the disk usage of the subdomain/additional domain use the following article: How to check disk usage for a domain/subscription/client in Plesk?