It's not a question of overeating... it's all about what you eat and being honest about it.
Case in point, my partner is on the Weight Watchers Diet. He does eat a LOT of food, and not the food that Weight Watchers sells. He makes a lot of his own salads and grills his own meats including chicken, fish, pork and sausages. In the last few months, he's gone down from being 210 pounds to 183 and he's still working at it. The reason why this diet worked for him is because Weight Watchers provides a system that allows people to count calories, as well as figuring out how many calories one can consume in order to maintain or lose a certain amount of pounds a week. Looking in the book, you find out that you can eat almost all the vegetables you want because most of them, regardless of how it's cooked, isn't much in caloric intake. Grilled & roasted meats like chicken & fish also don't have a lot of calories. In other words, I could eat 3 cups fresh spinach (21 calories), a cup of whole kernel, canned corn (180 calories) and a full size piece of broiled chicken w/o skin (142 calories). That's enough to fill most people and is a lot more food than a Whopper (588 calories), but considerably less in caloric intake. And if I followed up that big meal with 1 hour of vigorous weight training which burns 471 calories, I essentially eliminate it all. However, you could still eat the Whopper if you did the exercise and leave barely more than 100 calories left. And the body does need SOME calories in order to work properly.
I jokingly downloaded a small fitness app for my iPod Touch called "Lose It!" recently and tried to work with the system. All one has to do is input what you eat or drink during the day. It also keeps track of exercises that help you burn calories. It also keeps track of calories for things like mixed drinks and some of the most common fast foods, and even allows you to make your own meals or input new ones. In less than a week using the tool, I've already lost over 4 pounds. This even after having one night out at a few bars with friends, when I surprisingly found out that having just a few Long Island Iced Teas, a shot of Vodka and a bottle of Heineken was the caloric equivalent of having a Big Mac.
Even if people are ultimately responsible for what they eat, it's also a restaurant's obligation to produce accurate information about the calories in their foods so that people can be responsible. It's not enough to just say, "skip eating at McDonald's" when you can actually get some food there that is essentially lower in calories than some health foods especially when you don't have immediate access to a healthier choice. Their classic grilled chicken sandwich with no sauce has always been listed in top tens of one of the healthier fast food sandwiches, including Men's Health Magazine. Losing weight isn't as complicated as it sounds, nor does it require a sadistic food & workout regiment. But it requires an honest effort from everyone.