Outlook 2010 beta version had issue in terms of message size and few user had previously reported about this issue. This was a bug in the Outlook 2010 Beta where the usage of number and bullet lists causes redundant CSS definitions to be included in each outgoing message thereby increasing the message size. The Outlook team has released a fix for this issue and it is now available for download.
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. So in case you have messages already in your inbox with large size, you will need to clean them up manually.