Why Exchange Online Mailbox Size Is Larger Than On-Premises During Migration
Question details
The user needs to understand why an Exchange Online mailbox reports a much larger size than the source on-premises mailbox during a hybrid Exchange 2013 to Microsoft 365 remote move migration.
- Product
- Microsoft Exchange Online and on-premises Exchange 2013 hybrid migration
- Device & OS
- not provided
- Scenario
- A hybrid Exchange 2013 to Microsoft 365 remote move migration is in progress, and the administrator compares the on-premises mailbox size with the migration batch size.
- Observed behavior
- The on-premises mailbox shows only about 192 MB, while the migration batch reports nearly 13 GB synchronized.
Open the Exchange Admin Center or Exchange Management Shell with Recipient Management or Organization Management permissions, and identify the specific migration batch and user mailbox. Be aware that Exchange Online mailbox size calculations can include Recoverable Items, hidden folders, and data that the standard on-premises mailbox size does not show.
Compare mailbox statistics and check Recoverable Items in Exchange on-premises and Exchange Online
Use this first to identify hidden mailbox data that is counted in Exchange Online but not shown in the standard on-premises mailbox size.
Exchange Online and on-premises Exchange calculate mailbox size differently. Recoverable Items, audit data, and hidden folders can be included in the online mailbox total and migration data count even when the on-premises mailbox appears small.
Go to the Exchange Admin Center in Microsoft 365, select Recipients > Mailboxes, and open the affected user mailbox to view mailbox usage details.
Run Get-MailboxStatistics -Identity user@domain.com | Select DisplayName,TotalItemSize,TotalDeletedItemSize to compare regular mailbox data and Recoverable Items.
In the on-premises Exchange Management Shell, run the same Get-MailboxStatistics command for the source mailbox and note differences in TotalItemSize and TotalDeletedItemSize.
Check for hidden folders, litigation hold, retention policies, and the Recoverable Items folder that may store large amounts of synchronized data.
Review the migration batch status and understand incremental synchronization
Use this when the batch shows as synchronized but the reported data keeps growing.
Check Azure AD Connect logs only when directory synchronization issues are suspected
Use this only if you suspect duplicate objects or directory synchronization problems that could inflate the online mailbox usage.
Work with WPS Office for Daily Documents After Exchange Migration
While WPS Office does not change Exchange Online migration size calculations, it is a free, lightweight Microsoft Office alternative for opening, editing, and creating Word, Excel, and PowerPoint files after your Microsoft 365 migration. It preserves familiar formatting and supports seamless document migration for everyday tasks.
Frequently Asked Questions
Why does Exchange Online show a mailbox size of almost 13 GB when the on-premises mailbox is only 192 MB?
Exchange Online mailbox size calculations can include Recoverable Items, hidden folders, audit data, and other data that is not reflected in the standard on-premises mailbox size. Run Get-MailboxStatistics and compare TotalItemSize and TotalDeletedItemSize to identify the difference.
Does a status of synchronized mean the migration batch has finished copying the mailbox?
No. Synchronized means the initial synchronization is complete, but incremental synchronization continues every 24 hours. The reported data can grow until you complete or remove the migration batch.
Should I check Azure AD Connect when the migration batch reports a much larger mailbox size?
Only if you suspect directory synchronization issues or duplicate objects. If Azure AD Connect already reports SUCCESS with no corrections, focus on mailbox statistics, Recoverable Items, and the migration batch details first.
Can WPS Office fix Exchange Online migration mailbox size differences?
WPS Office is a document suite and does not modify Exchange migration size calculations. However, it can be a free and lightweight alternative for opening and editing Microsoft Office documents after your email migration is complete.
