PST file size not changed after archiving

Copper Contributor


in Outlook (Microsoft 365 version), I performed an archive of a 56GB .pst file by archiving messages older than 31.12.2018 to a separate .pst file.
Outlook created a separate archive file with a size of 17GB.

After archiving, I used the "Compact" option, after which in Outlook the old file (that had a 56GB) shows in the "PST data file properties" a size of 39GB, but in windows explorer the file still has a size of 56GB.

What could be the cause and how to update the file size in the explorer to the actual size of 39GB ?

1 Reply

Hi @CzkLTPR,

when emails are archived, the spaces previously occupied by the archived emails in the old PST file are marked as empty and are available for new data storage. However, these spaces are not immediately removed, as rewriting the entire PST file for each deleted email would significantly slow down the process.

Outlook's "Compact" feature is designed to eliminate these empty spaces. Nevertheless, the compacting process may not promptly reduce the file size, as it operates in the background during idle computer time. Consequently, the file size in Windows Explorer may not immediately reflect the changes after compacting.

To attempt further reduction in file size, you can rerun the compact process using the following steps:

  1. Navigate to File > Account Settings > Data File.
  2. Select the PST file.
  3. Click on Settings > Compact.

If the file size remains unchanged, it could be due to the presence of large attachments or other data within emails in the PST file. In such cases, manually deleting unimportant emails or large attachments and then rerunning the compact process may be necessary.

Outlook mailbox size not reducing after Archive - Microsoft Community

why doesn't the outlook.pst file get smaller after archiving - Microsoft Community

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