I am probably smaller than average and was picked out on a Europe-SA flight last year to move to sit between two very large people in the centre block of a cabin configured 3-4-3, as the person sitting between them, rather larger than I am, had complained that he was going to be 'squashed'. The manner in which the FA asked me to move was far from pleasant and as I had booked a window seat I felt that this was unacceptable and refused to do so.
My attitude is that I'm sorry for people who are extra large, either in girth or height, but regardless of whether they are like that genetically or through gluttony, it's not my problem and I won't be the victim of it.