Food density could not be the reason. Lions and tigers eat high density food and stay slim. Pegs and cows eat low density food and they have more body fat.

I think combining fat with carbohydrate could be one cause of the problem. Breaded and fried meats, french fries and chips make the worst examples.

When we eat enough amount of carbohydrates our bodies run entirely on glucose since it is the preferred fuel and fats are stored as body fat. Remember, when carbohydrates are available, we don't turn fats into ketones as we do when we are on a low carb diet.

One more reason for obesity in the west is that people continuously attept to lose weight by reducing their calorie intake. To our bodies, fat is a precious energy reserve which must be maintained. Each low calorie diet is interpreted as a famine and each time our bodies encounter a famine, they decide to make us more energy reserve after the famine is over!