This has happened to me on a few situations and the person did their best to bot invade my space. I had one lady who was so funny and obviously had faced this situation many times. I would have enjoyed her in any situation. How we handle it creates the context for the rest of the flight. I am not into body shaming anyone and I do not know their story...but we all have one.
Airlines would do us all a favor if they had a row or two designed with larger seats that accommodate people who have girth. Not saying they have to charge more - not my call but it would/ could create a desire for those who need more room to fly and be loyal to that airline.
I will see the good in people, regardless of their size.