Hi anonymous user,
Based on my experience, it's common for the exported PST file to be much larger than the mailbox size. This is because Exchange Server mailboxes store email more efficiently than Outlook uses in a *.pst or *.ost.
For more information, hopefully you can find the following article helpful:
Comparing Mailbox and PST File Size
By the way, I've seen several similar threads which also mentioned that the PST size is around 3 times huge on archive mailbox size, I'll leave the links below for your reference:
Online Archiving into PST took too much space
his online archive mailbox size is 48 GB but while I go to downloading his mailbox into .pst, it's .pst is more than 150 GB
Mailbox size much smaller than PST size
For those that still run their own Exchange, I usually see an average of 2-3x the size of PST vs actual mailbox in a stand alone user
(Please Note: Since some of the web sites above are not hosted by Microsoft, the links may change without notice. Microsoft does not guarantee the accuracy of this information.)
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