I have some experience of this and thought I'd share it with the thread. A fews years back I was flying what I'd call "medium haul"; a 4 hour flight during the day. I was flying economy and was sat towards the rear of the aircraft with 2 empty seats next to me.
I watched everyone get on and nobody came near "my" two emty seats. As a tall person I get quite uncomfortable in an economy seat (lack of knee room) and was looking forward to stretching out once we were under way. And then the 2 missing pax turned up. The reason for their late arrival was that they were clinically obese and had trouble making their way up the aisle towards the rear of the aircraft. I immediately knew these two people were destined to sit in "my" empty seats and had visions of a tortuous 4 hour journey with cramped knees and being squashed up against the cabin wall.
I immediately attracted the attention of the cabin crew nearest me and explained that there wasn't a realistic chance of the 3 of us sitting comfortably in our allocated 3 abreast seating. She agreed with me but pointed out that the flight was full.
I said I would refuse to take my seat and would rather deplane than endure 4 hours of torture. My pleas (and note: this was out of earshot of the two obese pax) were heard by two young girls, both slim and considerably shorter than me. One offered to sit in my seat as she thought that she wouldn't be as inconvenienced as I was surely to be.
Problem resolved we departed.
Upon my return I explained to an overweight colleague of mine what had happened. She pointed out that she always booked 2 seats as a) she didn't want to inconvenience others and b) she didn't want the embarrasment of someone else complaing that she had "overflowed" on to their seat and c) it was more comfortable for her, especally when trying to eat the inflight meal.
Some airlines now ask their larger pax to book 2 seats!
I think the issue here is about not inconveniencing other passengers. If the person sat next to an obese pax is not able to fully enjoy their seat (i.e. they have to lean away from the obses pax or are squashed by the obese pax or they have permanent skin-to-skin contact with the obese pax that they would rather avoid) then it is up to the obese pax to book 2 seats.
There is no need to call obese pax lardy-arses or anything else. They are obese, just as I am tall and others are short or under-weight. However there is a right for pax that fit into their seats to complain should they have to share that seat with the pax sitting next to them.