One possible problem is that messages are not being deleted from the server when you check email.
To double-check your Outlook settings... Open Outlook then:
Tools -> Account Settings -> (click on the account that you want to check) --> Change --> More Settings --> Advanced --> Make sure that "Leave a copy of messages on the server" is UNChecked.
The WebMail Sent and Trash folders are another possible source of messages that may be filling up your mailbox. If you have used Webmail to send messages, log into Webmail and clear your Sent Messages folder. Also be sure to actually Purge your Trash folder, otherwise the messages that you delete with WebMail are still sitting in the Trash folder taking up space.