Has to be something about the "size" of the club. I noticed Redcafe is very self-conscious when it comes to that. So here it goes:
Real Madrid, Barcelona and Bayern are clearly bigger clubs than Manchester United, based on European Cups, league titles, current success, historic teams, Ballon d'Or winners etc. etc. and it's arguable for Juventus and Milan as well, although the Italian sides are unlucky to be playing in a broke league right now while United is lucky to be playing in the richest and most marketable league which automatically keeps United financially on the top and well-marketed irrespective of the actual success on the pitch but you wouldn't call Arsenal bigger than 7 times European Champions Milan either just because Arsenal are richer, better marketed and have more people following the Premier League right now, would you?
And yes, that was one sentence.