I think there is one very important aspect to this: the person sitting next to the "person of size". When a person buys a seat, he or she is buying space. It is not ok if an overweight passenger intrudes into the space purchased by another person. That includes legs and the arm rest. If the overweight person can't fit into his or her allotted space, buy two seats or a higher class of service with more space. It's that simple. You are buying space and you can't use more space than the fare you purchase affords you.