When you aren't fat, I assume that you look at a fat person eating a cookie and say, "how can they do that, knowing that they are so big? It isn't that hard to eat carrots and apples?" But, do you know why they eat that way? Have they always run to food for comfort because of a poor upbringing? Did they have parents who never forced them to eat healthy food, therefore they don't like it, or won't taste it? Maybe they are trying so desperately to lose the weight and watch what they eat and they can't help but be tempted by the food court or the ice cream shop?
It is a constant battle. What seems so easy for you, might not be so easy for the next person. And yes, you are right, it didn't happen to a fat person overnight. But your judgmental glances or the "watching" them eat and shooting devilish glances in their direction until they become so uncomfortable they get up and leave, does not help. Don't say that it is "gross" to watch them. It is just as gross to watch you eat chili cheese fries too. Don't laugh at them or snicker behind their backs at the gym when they are so desperately TRYING to do something to better their lives. When did judging someone give you any gratification?
Just remember, they do have feelings, be supportive, not judgmental and then maybe, someday, we'll see a difference.