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Why Outlook Migration Folder Sizes Differ & How to Verify Data
Question details
Users want to understand why their email folder sizes shrink significantly after an Outlook-to-Outlook migration and how attachment storage affects this.
- Product
- Microsoft Outlook
- Device & OS
- not provided
- Scenario
- Migrating mailboxes, folders, and attachments from one Outlook account or data file to another.
- Observed behavior
- The destination folder shows a much smaller total size (e.g., dropping from 11.6 GB to 7 GB) than the source folder, raising concerns about missing attachments or data loss.
Before troubleshooting for missing data, ensure the migration process is fully completed and review your migration tool's logs for any skipped items or synchronization errors.
Compare Item Counts and Verify Attachments
Confirm whether the size difference is due to expected structural compression or actual data loss by checking item counts.
A difference in folder size does not automatically indicate data loss. During migration, data is often restructured, compressed, and de-duplicated. Rather than relying on total gigabytes, you should compare the actual item counts between the source and destination.
Right-click the source folder in Outlook, select 'Properties', and note the total number of items (emails, contacts, etc.). Repeat this for the destination folder.
Sort your destination inbox by 'Size' to locate emails with large attachments. Double-click to open a few of these attachments to confirm they were not corrupted during the transfer.
Open the logs generated by your Exchange admin center or third-party migration tool. Search for 'failed', 'skipped', or 'excluded' flags to see if specific files were left behind intentionally based on migration policies.
Understand PST vs. OST File Storage Differences
Recognize how local archives (PST) and offline data files (OST) handle metadata and attachments differently, affecting total file size.
Need a Lightweight Office Suite to View Exported Attachments?
If your email migration involved thousands of document attachments, you need a fast and reliable office suite to open them. WPS Office offers a free, lightweight, and fully compatible alternative to Microsoft Office, easily handling all standard document formats without slowing down your computer.
- 1. Download and Install WPS Office: Visit the official WPS website, download the installer, and follow the on-screen prompts to complete the installation.
- 2. Set as Default Application: During setup, choose to make WPS Office the default program for Word, Excel, and PowerPoint files.
- 3. Open Email Attachments Instantly: Double-click any document attachment directly from Outlook or your email client to open it rapidly in WPS Office.
Frequently Asked Questions
Does a smaller folder size after an Outlook migration mean I lost my emails?
Not necessarily. A smaller destination size is extremely common and usually results from data compression, the automatic removal of duplicate items, and more efficient metadata storage in the new environment.
Why do PST and OST files show different sizes for the exact same emails?
PST files are local archives that tend to accumulate fragmented data and 'white space' over time, causing file bloat. OST files sync directly with Exchange or Microsoft 365 and use more advanced compression structures, leading to a smaller footprint for the exact same content.
How can I be absolutely certain all my Outlook attachments migrated successfully?
The most reliable method is to compare the total item count (number of emails) instead of the total file size. You should also check the official migration logs for skipped items and manually open a few large attachments in the new mailbox to ensure they load properly.
