Or we could get rid of anti-discrimination laws and allow airlines to charge based on weight/have assigned seating types. But I imagine that wouldn't go over very well with the people who want to ban junk food.

Most airlines in the US have an official policy allowing them to require passengers who cannot sit in a single seat with the armrests lowered and seatbelt fastened to pay for an additional seat. They usually try to employ alternatives, but on full flights, people do get bumped if they don't fit in one seat.

I'd be fine with both. Where's the conflict?

Huge difference. One allows free association, whereas the other involves using government force to take away freedom.

Those are arbitrary "principles" that only make sense in the context of a given arbitrary political ideology.

I want to live in a country where it's easy to be lean, so that as many people as possible are lean, because there are no downsides to being lean, and infinite upsides.

Paying in relation to what you use seems fair. Sending small packages costs less than big heavy packages. What's a few bucks' difference anyway?

There is no such anti-discrimination law in the USA. Obese people aren't a protected class.

Unfortunately, it has been pursued as though it is, which is a natural progression of the 'slippery slope' argument. For example, there was the relatively high profile Hooters weight discrimination lawsuit which was allowed to proceed and settled for an undisclosed amount, even though there was no legal basis of wrongdoing.