Online Archiving into PST took too much space

Question

Dear Experts,

We're facing an issue, we're using in place exchange 2016 and enabled one user to use exchange online archiving feature, 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, what could be the reason?

Answer 1

Did you export the online archive mailbox using the New-MailboxExportRequest cmdlet?
If this is the case, usually the size difference between the mailbox size and the exported .pst file size could be related to the Recoverable Items folder. (The archive mailbox contains its own Recoverable Items folder, see this article.)

Please run the command below to check the archive mailbox size with items within the Recoverable Items:

Get-MailboxStatistics <UserIdentity> -Archive | Format-List StorageLimitStatus,TotalItemSize,TotalDeletedItemSize,ItemCount,DeletedItemCount  

You can add the mailbox size and the recoverable items size, then compare the result with the .pst file size to see if they are closer.

Update:
In addition, based on my research, Exchange database stores messages more efficiently, so it could be normal that the .PST file size is larger then the reported mailboxes size. Here is an relevant blog for your reference: Comparing Mailbox and PST File Size.
A comment by JARITM in the thread below also mentions that "I usually see an average of 2-3x the size of PST vs actual mailbox in a stand alone user":
Mailbox size much smaller than PST size
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