I was on a flight to Nassau and I am glad the plan was empty because the woman who would have sat beside me took up about 25% of my seat.
Either that or negotiate with a skinny person to pay for the 25% of your seat they are taking.
However, in the case of a 435# guy, I think they would be taking closer to 50%. Even Twiggy couldn't fit on half a seat.
I no longer sit next to my husband on a plane, if sitting next to him means occupying a middle seat. My husband is not over weight, but he is 6’6”, and has very wide shoulders. After being wedged between two big men in an exit row, and having to put my seat back far enough to allow me to sit back, I gave up and sit by myself.
One time this summer, my husband asked for an exit row and was seated in a middle seat, next to a big man with Tourette’s syndrome.