7.3 For all passengers

I agree with the situation that obese people must sit on big seats and they should pay extra charge. But, who is the irregular people for airlines? One policy is necessary for airlines to make rules for obese passengers. Each airline has each standard for fat customers. For example, according to Southwest Airlines' website, they need customers who cannot seat economy seats to reserve two seats for their width. They declare the width fir each seat is 17 inches. Also, United Airlines has its own regulations for obese customers. They want passengers whose size is bigger than armrest size to buy two seats. They do not say the width clearly. On some another airlines' websites I could not find such rules, but it is said that every airline has their own rules for obese people.
The reason that airlines want extra charge for fat people is to keep them safely, even if the reason is just outward. So, they should decide the same standard for such people, or customers would wonder if they can be regular people for airlines or not.