Having the cheapest possible flights is not a civilization goal we should be aspiring to, especially in light of climate impacts.
No they shouldn't be. That would mean way fewer seats would fit on any given plane, which would send ticket prices through the roof for everyone. People who aren't fat shouldn't have to pay extra to accommodate those who are, when the options of buying two seats or a first-class seat are available.
> Having the cheapest possible flights is not a civilization goal we should be aspiring to, especially in light of climate impacts.
Climate impact is even more of a reason to not do what you want! We'd need more planes and bigger planes for the same amount of air travel, and so the carbon footprint of any given plane ticket would be much higher.
But everyone does have the same body type. They are just not all the same size. What you're saying is basically that people who are disadvantaged by their size should be given priority over those who are disadvantaged by being poor.
I would like to live in a world where people who are unusually big because of genetics or a genuine disability or medical condition or whatever can have the same experience as everyone else at the same price. And if that was truly the cause, we wouldn't be hearing about this. Airlines would just eat the cost and avoid the controversy. We're hearing about this because it's a massive epidemic in the West, brought on by lifestyle, and the airlines can't afford to assume that overweightness is an anomaly any longer.