You don't have to be fat, to encroach on someone's personal space!
Earlier this year I was on a flight from BKK-SYD (9 hours), next to the biggest man i have ever seen in my life. He was not fat, but really well built, and he was about 7 foot tall. His bum was not that big, as with most body builders, so no problem in getting in and out of the seat, but his upper body was about twice the width of the seat. I had 9 hours of a bicep the size of a watermelon next to my face! He definately needed two seats! Or should perhaps save the money he spends on steroids
, and use it to buy a business class ticket!