There is a relationship between mailbox size and data transferred but they can be quite different and cannot be compared directly.The size statistics shown on the mailbox statistics page indicates the amount of data transferred over the wire. This typically differs from your mailbox size (as reported by your Source email system) for different reasons including:
- Size calculation: Your Source email system will calculate and report mailbox sizes using their own proprietary formulas/methods.
- Content conversion: The same data may be encoded using different formats at the Destination (i.e., plain text + HTML MIME parts).
- Protocol overhead: Data as read from the Source will often include protocol data such as headers, flags, IDs, etc.
- Data encryption: Unless you configure your connection to use non-SSL endpoints, all data will be encypted.
- Data compression: Data may be compressed or decompressed during storage or transmission.
- Data encoding: Many communication protocols will encode data (like Base64 encoding, SOAP encapsulation).
Also, some items may not have been migrated (details), or may have failed to migrate (up to the error count specified on your project.
If you are trying to compare mailbox sizes because you are looking for missing items, click here.