I must admit that it is not such a bad idea as long as they (an airline) take into account the height etc, ie use a "body mass index" rather than just weight. These fat people cost more to transport than slim (normal) people so they should pay for it.
Good incentive to slim!
I also think that these people, along with smokers and drunkards, should pay more NI contributions to cover the huge extra costs of their NHS usage.