The heavier a plane is, the more av-gas is needed to fuel it.
The more av-gas is used to power a plane, the more expensive it is to run.
Therefore it makes sense for Samoa Air to charge passengers by weight rather than by seat.
The airline's decision to charge passengers according to their weight has prompted a mixed reaction, but a pay-as-you-weigh system seems perfectly fair to me.
I'm not coming at this from a position of righteousness, either. I'm at my all-time heaviest weight and the husband is a 1.95m slab of man, so it would cost us a fortune to travel if pay-as-you-weigh was adopted across all airlines.