Exchange 2019 excessive archive size
Solution 1:
Have you checked the items in the archive mailbox?
I suppose there should be many duplicate items.
Below is a Microsoft KB about the similar issue,while it is about Exchange Online:
Items reappear in a user's mailbox or are duplicated in the archive mailbox when an archiving policy is enabled during IMAP migration
In this case,would it be possible that the primary mailbox on Exchange server is syncing with some clients?