But I think you overstate the idea that we buy a "seat" on an airplane. That's just a practical way of selling the right to board the plane because the airline must put passengers in seats (can't be standing around like on a city bus) and because they cannot have more people on the plane with tickets to go from location X to location Y than they have seats.

You extend that purchased "seat" idea to all the space around the seat. I agree that there are social norms about the space I should and shouldn't occupy. But those are social norms, nothing contractual. In other words, I don't think we need to change the way we sell access to public airline transportation all in an effort to enforce these social norms. I don't think we need to force those who (inadvertently) violate those social norms (because of their size) to pay more to access public transportation. It may be annoying when they are in our space, but that's life.

The fundamental issue is whether your airfare entitles you to more than just a seat; whether it entitles you to the air space around your seat. I personally don't think it does, and believe those are just social norms that need no formal enforcement. Those who believe they purchased the air space around their seat, or who believe social norms should be formally enforced, will obviously disagree.