As many beta users discovered, Outlook 2010 sometimes creates huge messages. This is a bug and is usually only a problem when using bulleted and numbered lists. It’s fixed in later builds.
The good news: Public beta users won’t have to wait for the release of a later build. Microsoft released a hotfix beta users can download.
For more information, see:
Issue with message sizes in Outlook 2010 Beta
Published February 12, 2010. Last updated on July 18, 2014.