If airlines increase seat width to accommodate overweight, no of seating in aircraft will go down, which will lead to increase in per seat pricing, in effect healthy people will be subsidising overweight.
It decreases seating less than you might expect. Seating on an aircraft is usually limited by the number of emergency exits rather than actual dimensions.
I could see that being the case for pitch, maybe (though much further reductions than some airlines have done seems likely to cause medical issues for tall people). But width? The only place for that to come from is from the aisles and typical 3-3 or 2-4-2 layouts don't leave very big aisles (and the results would not be navigable for anyone using that extra seat width)
I was on a flight just yesterday, placed in a middle seat, where the two gentlemen flanking me rolled over the arm rests into my seat space. As an unhealthily slender man I am 100% comfortable subsidizing not having my seat space invaded by people who have a different shape than I. I don’t want to pay extra to personally avoid it, as I could in first class, I want to pay extra to make sure nobody suffers either side of the problem: these men were both profusely apologetic and embarrassed, and I’m sure I was not the only of us three left also physically uncomfortable.
No, even __bone structure__ width, shoulders, knees to hips. Airline seats seem sized to accommodate a hypothetical 'average' adult; not the maximum of any given dimension for a even a healthy adult.
If you don't account for leg length, or lack of thereof which affect large people more than seat width, any not morbidly obese adult fit in an Airline seat. You may have a distorted view of what consist an healthy adult. And no people spending a lot of time lifting weight to grow muscles more than the typical crossfit athlete would are not healthy. They lose so much flexibility they can't do many simple thing, let alone scratch their back.
The fact is there are many non healthy adults and the question is what do we do about them. Is flying a fundamental right? Should people with over the top obesity have to pay more to use 2 seats? Should large people have to pay extra or business class tickets to get a decent leg length?
It's probably more efficient to just get two neighboring seats, lift the arm rest and get a belt extension. Chasing maximum sized people seems inefficient. Imagine designing a plane with each seat having to fit someone like Brian Shaw.