Hello Salem,
I would start by verifying where the disk space is being used ( Like Zach mentioned above ), is this all Email or does it include File Storage, etc.
Select the mail tab -> Than the info circle at the bottom next to the size, it will show where this is being used.
Once this has been verified that the usage is in the mailbox you can move on. IN outlook are you seeing any "Sync Issues" that may indicate that all the messages are not being downloaded? Are you using compression on the Archive file?
You can try and import the archive into a test user and see if the message count is correct for the folders.
Thank you
Tony
Tony Scholz
System/Network Administrator
SmarterTools Inc.
www.smartertools.com