It’s straightforward enough.
When you turn up at the airport with luggage to check in, if it is over the limit (which may be included in the ticket price or charged additionally) you are charged extra per kilo.
So it should be with people. As you check in you stand on a scale. If you tip it over the airline’s prescribed limit you pay extra. Furthermore, if you are excessively overweight you must purchase two seats and this should be made known under the booking conditions. Those who are so big as to encroach on the next seat and who have only paid for one will be denied travel.
Yes, there’s details to iron out, but in principle that’s what should happen. Nobody should have their seat invaded with other people’s bodies.
I sympathise with you Andy, re the geezer in a vest. Strict codes of dress should be applied on aircraft: No sleeveless vests, no shorts, no flip-flops, no bare feet. Also no taking huge amounts of clothes to change into during the flight (thus blocking the kharzis for ages). I’ve travelled all over the world and nowhere warrants you having to dress for the beach when arriving or departing hot climes. If it is/will be cold at the other end, take an overcoat and put it in the overhead. Airlines need to ensure that their passengers behave in an acceptable manner. One of the most annoying things is the amount of junk people insist of carrying on as hand luggage. Changes of clothes (unnecessary – see above); mountains of food (absolutely no need on short haul, airlines feed you on long haul); piles of books, electronic gadgets, crossword puzzle books. You name it. Should all be stopped