I 've seen numerous instances where this folder contains 50% of the size of a mailbox. My recommendation is to delete the messages in here, then re-calculate the overall size of your mailbox to see if it reduces the size as expected.
Finally, if neither of those ideas worked I would right-click Mailbox in Outlook and choose Folder Size. From there, meticulously scan through the size of each folder. You'll probably find some surprises along the way, but by stepping through the list of folders and associated sizes you should be able to find the source of your problem.
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