Technically both are correct. The more food you eat, the more energy you get. Professional athletes eat a lot of food because they have high intensity work outs every day and they need a lot of energy. If however someone eats a lot but doesn't use the energy the food offers, then our bodies create adipose tissue, fat, which essentially stores this energy to be used at a later time. The problem is that if you live a sedentary lifestyle and you eat a lot you never use up this fat, since the body will always prioritise freshly consumed food over fat deposits, so in order to lose the fat you either need to require more energy than your food provides, as in working out but at the same time regulating how much you eat, or not eating at all, which is dangerous to do since aside from energy we also get nutrients and vitamins from our food.

Moreover how much fat your body creates and how much it burns is also affected by other factors since everyone's body and metabolism may work differently. Some people get fatter easier than others. In general it's good to have an active life style and walk often, work out at least a few times a week. Not everyone has to be a ripped demigod but a sedentary lifestyle can be really damaging to health, and becoming obese on top of that makes it even worse since it contributes to a host of health problems, mainly heart problems but also joint problems, respiratory problems, dermatological problems and many others.