If you are using Outlook 2010 Beta, you probably need this fix because all of your emails are huge. It is fixed in the recent RC, but most people don’t have access (it’s not a public RC). The Office team has just posted a fix for this issue for folks using the Beta.
Recently, we posted information on the Outlook blog about a bug in the Microsoft Office 2010 Beta that increases the message size of some e-mails messages sent from the Microsoft Outlook 2010 Beta. The Office team has been hard at work building a fix for this issue, which is available immediately.
This fix prevents future messages from consuming unnecessary space, but doesn’t resize existing messages. If you reply or forward an existing message that is affected by this bug, it remains enlarged. To help reduce the impact of large messages on your Inbox and mailbox storage quota, you can also run Conversation Cleanup (new feature in Outlook 2010) on large folders. On the Home tab, in the Delete group, click Clean Up. Also, consider starting new message conversations or threads when possible.
(Source: MSDN blogs)