Actually, fat is the most energy dense of any food type. 1 gram of fat=9 kcal, 1 gram of carbs or protein=4kcal.
The obesity issue, is not well understood by many and what people eat and what they eat, tends to be the dominant reason many apply to the dramatic rise in obesity over the last several decades.
The fact is; in the US, we actually consume less fat today, than we did decades ago, but we are far more obese. Sugar consumption has gone up some as well. Sugar is bad, because it causes the most dramatic rise in insulin in the human body and since insulin's job is to take glucose out of the blood and either store it in the liver, in the muscles as glycogen and or in fat cells. When the amount of sugar consumed, exceeds the first two physiological needs, the excess gets stored as fat. So, on to the other most dramatic change in lifestyle over the last several decades; the percentage of people, who live sedentary lifestyles. The development of; computers, video games and television entertainment, has many more people sitting for much longer periods of time, than we saw decades ago. The human body was not meant to be sedentary and if adequate levels of physical activity are not achieved, that sugar goes towards one thing; promoting fat storage, because there is no physical demand on the body, to burn up the sugar and or to store the sugar, as glycogen for future physical activity.
IMO, lack of physical activity, is the number one reason obesity has climbed as it has. Diet is important, but diet alone becomes less crucial, the more active the human body is, because the calories get turned over and burned.