From what I can tell as a WP7 user, the Facebook app's navigation takes up more space than other apps. Those "tabs" that look a bit big are not usually "dead space". In Microsoft-developed apps (Office and Music, for example), the tab titles slide up and away as you go through vertical content. So all that's left is the logo at the top. I've been using a LG Optimus for a year now, and I've never felt that the interface was cramped, so maybe it just looks odd to a non-user.