The Outlook Team has put out a warning about large message sizes when sending out a message with Outlook 2010. While working with a Beta version of software could always lead to unexpected results, this bug can also affect the recipient of the message.
The issue is related to using bullet lists. When you use bullet lists in your message, the CSS of the message (which controls the formatting of the message) does not get properly truncated and a small message could become quite big. This could even result in that the message may no longer be displayed correctly at the recipients end.
The current fix is to simply restart Outlook 2010 Beta (at least once every day). This issue will be fixed of course once Outlook 2010 gets released.