My instinct is that webmail, mostly in the hands of Hotmail and Yahoo! mail, has long eclipsed local email programs (like Outlook) in the minds of most users in the world. Gmail was a late comer to this game, and doesn't have nearly the users they do; when you think about 500 or 600 million people the world over using Hotmail and Yahoo! mail, it's hard to believe that local email programs even come close.
Edit: sorry, rereading, now I realize maybe you mean Microsoft has far more users than Gmail. Which is absolutely true - there are around three times as many people using Hotmail as there are using Gmail.
As for Outlook, CampaignMonitor's data shows that Outlook edges out webmail by quite a lot [http://www.campaignmonitor.com/stats/email-clients/]. Not sure how accurate their data is, but it's better than raw speculation.