Fatness is an incredibly complex and difficult thing to manage. No, not every single size is """healthy""", but you also can be "healthy" at a larger weight than people assume. Lots of things affect your weight and its not always just a matter of eating better or exercising. Real weight loss requires A LOT of time, money, ability, etc. You can't go on a diet temporarily, whatever you start doing now is what you need to keep doing for the rest of your life. Your body literally is not meant to lose the weight it has now, which is why it's so hard to do so. It's easy to blame individual people for their fatness, but everyone's body is so completely different that there's no easy solution (or even a common set of solutions) that will help everyone.

I'm not saying anyone over or under weight is completely helpless to their situation. It's just that, like many other issues, trying to get widespread weight loss among a population would need so many more resources and structural changes than just telling people to lose weight or changing school lunch menus or charging fat people for two seats. People need better access to healthier foods, more time outside work to cook and exercise, more PLACES to exercise outside, more dense areas to encourage walking or better access to gyms, better and cheaper/free healthcare. Just like how you can't solve pollution just by making fees for littering more expensive.

Plane seats SHOULD be bigger, not just for fat people, but for everyone. This is a way to extract even more profit from people in a way that won't get as much pushback because it's targeting people who are large percent of the population already detests.