This was an April Fools joke (and presumably a dig at obesity in Samoa) that refuses to die. https://en.wikipedia.org/wiki/Samoa_Air ceased operations in 2003.
As much as I hate sitting next to a whale who is trying to occupy half of my seat, according to http://en.wikipedia.org/wiki/Boeing_737 a fairly typical jet while empty is about 62,000 pounds, and can carry a maximum of 124 people (in most configurations less). That's 500 pounds of plane per seat.
For the vast majority of people, your share of the empty weight of the plane exceeds the weight of you and your luggage by a good margin.
When you add fuel, you have to add (exponentially!) more fuel to lift the fuel you're adding. It's true that a 200 lb passenger doesn't cost twice as much as a 100 lb passenger, but it's more than the 17% increase one might expect going from 600 to 700 lb dry weight.