Microsoft Online Admin Center - Exchange Online

The Microsoft Online Admin Center (MOAC) Exchange Online displays the Online Company's Mail Storage and allocated storage for all mailboxes provisioned for users.  In reading this information note that each user that receives an Online license, receives a 1GB mailbox.  If your company is provisioned with 5 Online User Licenses and those 5 licenses are assigned to individual users, the Allocated Storage for all Mailboxes wil display 5GB of 5GB in Use.

This setting doesn't display the amount of disk space used, either by an individual or a sum total for all mailboxes.  It simply displays the upfront allocated space granted to users in the company.  To determine mailbox size for individual users, they must go into their Outlook Mailbox and look to the user's Inbox Properties for the total size of the mailbox.