Bigger seats are available in premium economy/ upgraded sections.
If, as Sassy suggests, we all pay a little extra for each cabin to have a couple of larger seats for people who need them, then how would this allocation be decided? Would it be first come first served? Or would need be prioritised? E.g., would pregnancy trump obese, or would 180kg trump 170kg?
If it's first come first served then there would still be the potential problem for larger passengers to end up in smaller seats, and therefore encroaching on other passengers - who may then be put out to find that they have paid $20 extra for someone else's seat, only to find that they still have a larger passenger who is taking up some of their space.
So do we then say that if there are those extra larger seats (that we are all paying extra for), then larger passengers are no longer allowed to travel in a seat that they do not fit properly?
Or do we say that larger passengers will need to pay for premium economy (or higher), or not travel at all (if they cannot fit into an economy seat)?
I'm not trying to pick holes in your suggestions, but it just seems like a very difficult thing to work out.
Another factor to bear in mind is that some of these problems (not fitting in to the seats) are more likely to be a problem for some races more so than others. It is likely to be a rarer problem for some of the Asian airlines (for example) than, say, the American carriers. (Just to pick a couple at random, where average body sizes tend to be different on a racial basis).
I guess my point is that it seems to be asking a lot of all airlines to make big (potentially costly) changes for a problem that is not the 'norm', and even less so for different demographics of people.