The Outlook team recently reported an issue on their blog about message sizes in the Microsoft Outlook 2010 Beta. A bug in the Microsoft Office 2010 Beta increases the message size of some e-mail messages sent from Outlook 2010 Beta.
The Office team has been hard at work building a fix for this issue, and it's now available on the Outlook blog. Note, the fix does not resize existing messages. To download the fix, and get some tips on how to reduce the mailbox size, head over to Fix for issue with message sizes in Outlook 2010 Beta on the Outlook team blog.