Yeah...the US airports are at a disadvantage here because there's so much domestic traffic. Even LHR has to deal with a certain amount of domestic connecting traffic and a lot of endpoint traffic.

Dubai's big advantage is simply that it is a major transfer hub with probably over 95% of traffic being international...and the vast majority of that, in turn, being long-haul (which means 747s, A380s, and the like) rather than shorter-haul (which London has a lot of to/from Europe). Basically, Dubai is ahead in passenger count because the airport handles a lot of through passengers (who get counted as both a landing and a departure in a given direction) in bigger jets and doesn't have nearly as much local traffic to compete with as most of the other airports.