What’s the Real Size of Exchange Online Mailboxes?

The Office 365 Substrate uses Exchange Online mailboxes to store a lot of data that users never see. The data is used by the substrate for different purposes, mostly to make it easier for features to get to relevant information. Microsoft doesn’t document exactly what is stored, where it is stored, and how it is used, so we must poke around in the innards of mailboxes to see what we can discover.