During archiving of items from a PST file, the size of the files may grow by 10-25%. This is normal behavior and must be accounted for during the execution of the job.
Microsoft designed the PST file format to be a database. Consequently, it has many properties of a database which may lead to size expansion even when opened strictly for reading. Some reasons for this are listed below:
- Outlook versions after the February 24, 2009 roll-up will automatically include new data localization optimizations in any PST file created prior to that date. These optimizations were intended to reduce disk access due to fragmentation within the PST. This alone can account for ~20% increase of the PST file in those cases.
- PSTs will use a special location within the file to manage resource locking. This permits access by multiple processes from the same user, but prevents simultaneous shared access amongst multiple users.
- The PST file will store the last applied user view (sort order, columns, etc.).
- If a search is performed on the contents, a temporary folder is created to contain the results of that query.
- As in any respectable database, the operations are immutable. This means that in-place modifications are never performed on the data. Instead, the data is copied to another location, and is modified and verified before the original is marked to be purged.
- Outlook will pre-allocate some free space (if there isn’t enough already in the PST) to reduce latency of the next user action.
Freshly exported PSTs generated by commercial software will generally expand the most upon reading, as they are the most dense when created. After the initial growth, however, any further increases should be minimal.