Whether the weight comes from obesity or from simple body size has no effect on the cost-per-kg to transport someone by air. If you're 6'5'', you're transporting more mass - it's only fair to pay more, because it costs more to move you.
As someone who is 6'8", 310lbs and spent 12 hours flying last week, I would have to strongly disagree. Until airlines provide a means of adjusting available room to compensate for height, its not appropriate to charge taller people more at all.
They do already, it's called first class and business class. The seats cost more and you get more room.