Something a lot of people don't realize is the whole "metabolism" thing. I'm not super overweight or anything but I could definitely stand to loose a lot of pounds. The thing is, I tried to last semester by going to the gym 4 times a week and doing cardio and the weight machines. I gained an enormous amount of muscle and kept the fat. I actually gained weight. Why? Because of my diet. Even though I don't eat much more (if any more) than the normal person, I am still overweight.
You can give the preach job of how I wasn't "working out" right and how I didn't do my diet right, but in the end, some people can eat all they want and not work out and still keep a nice body. People like me have to bust their ass to keep from being Class III Obese and eat celery all day to lose weight.
So when you see someone who is overweight, don't assume they are lazy and don't care about the person sitting next to them on a flight. It's not always that simple
Some people will always be bigger than the next person.