Agree that our sedentary lifestyles are a significant contributing factor to obesity, but you have it backwards about portion sizes.
Calorie dense (palatable) foods trigger our the reward centers in our brain more than non-palatable foods, and actually make us more hungry. This is an evolved survival strategy.
If you look at hunter gatherer diets, they will eat huge portions of extremely high calorie foods when they are available (e.g. drinking whole glasses of honey). They don't put on a lot of weight because they are more active, and don't have daily access to high calorie foods.
This means that large portions (and by extension obesity) are caused by the easy availability and low cost of high calorie foods. Telling people to eat less doesn't work because the systems that trigger hunger and fat retention work at a subconscious level and are in general more powerful than the conscious mind. The food environment that we have built up around us is a bad one for human beings.
If you want to learn more about how our hunger and fat regulation systems work then I can highly recommend "The Hungry Brain" by Stephan Guyenet .