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Why Microsoft 365 Mailbox Storage Differs from Gmail After Migration
Question details
Understand the discrepancy in mailbox storage size between a Google Workspace (Gmail) account and a newly migrated Microsoft 365 mailbox.
- Product
- Microsoft 365
- Device & OS
- not provided
- Scenario
- Reviewing mailbox storage usage and data integrity following an email migration from Google Workspace to Microsoft 365.
- Observed behavior
- The total storage space used in the migrated Microsoft 365 mailbox does not match the original mailbox size reported in Gmail.
Ensure you have global administrator or Exchange administrator privileges in Microsoft 365 to access migration logs and storage reports.
Investigate Migration Logs for Skipped or Errored Items
Check the official migration reports to ensure all data was successfully transferred without hitting API limits or errors.
During a migration, Google Workspace API throttling or unsupported item types can cause certain emails or attachments to be skipped. Reviewing the migration batch logs will clarify if missing data is the cause of the storage discrepancy.
Log in to the Microsoft 365 Admin Center, navigate to the left sidebar, and click on 'Exchange' under the Admin centers section.
In the Exchange Admin Center, go to 'Migration' to view your recent migration batches.
Select the specific Google Workspace migration batch, click on 'View details', and download the migration report for the affected user.
Open the report and look for the 'Skipped Items' section. Identify any emails or attachments that failed to migrate due to size limits, unsupported formats, or Google API throttling.
Account for Platform Architecture Differences
Understand how Google Workspace's label system and Microsoft 365's folder system calculate storage differently.
Contact Microsoft Support for Deep Investigation
If migration logs show a 100% success rate but the storage difference is massive and unexplained, escalate the issue.
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Frequently Asked Questions
Does Microsoft 365 count attachments differently than Gmail?
Yes. Microsoft 365 and Google Workspace use different MIME encoding methods for attachments. This difference in backend encoding can result in attachments taking up slightly more or less storage space after they are migrated.
Why is my Microsoft 365 mailbox larger than my Gmail mailbox?
This usually happens because Gmail uses a label system where one email can have multiple labels. When migrated to Microsoft 365's folder-based system, that single email may be duplicated into multiple folders, increasing the total mailbox size.
How long does it take for Microsoft 365 storage to update accurately?
After a large migration batch completes, it can take up to 24 hours for the Microsoft 365 Admin Center and Exchange Admin Center to accurately calculate and display the final mailbox storage usage.
