This is a most weird issue.
I have a couple of exchange OST mailboxes, and just for security, I am exporting them using File / Import / Export to a file / Export to PST file. If I run the export consecutively, it always creates files with different file sizes, WITH NO ERROR OR WARNING that something went wrong. The files should be of the same size as you run it right after the previous backup finished.
I found out that if the filesize is substantially lower, then a reboot and back up can fix this up.
What's your insight into this problem? What could cause that the files have different sizes and what could have caused that there is no warning?
I suspected some Windows Search issue as sometimes the backup fails with a dialog error stating that Windows Search terminated the export.