The problem with that, pistongone, is that in a 747 you get a reasonable average over a few hundred people. And a 747 can carry a few more tons; I doubt anybody would notice an extra 1-2% on the weight.
Whereas in a 2- or 4-seat spamcan you are averaging over far fewer people, and if you get 2 140kg fatties and 2 normal persons then you could be dramatically over MTOW.