Airlines are selling transport priced based on per person, not based on weight or girth. Even you acknowledge this. When a person buys travel for a person, they have the right to expect the seller to honor their end of the bargain.
If the airlines aren’t willing to properly serve all of their customers, that’s their problem, not the problem of the customer.
If they wish to discriminate based on size, they need to be up front about it and demand height, weight, and measurements at time of ticket purchase. At that point their booking system can sort customers into seats that fit them. They can even charge a slight premium for the plus-size seats, just like clothes, another good or commodity sold on a per person basis. Except that clothing manufacturers know that one size does not fit all.
The above would be an imperfect and non-ideal system but it would be leaps and bounds ahead of “Let’s tell people they’re buying travel for people — 4 people? 4 tickets! — and then later enforce that we treat a ticket as a temporary deed to 17 inches.”