Calorie or energy density is how tightly packed the calories in a food are. The two most important factors that determine the calorie density of a food are fat and water content. Fat has a high energy content, water has zero energy content. Dry foods with high fat content (such hard cheeses) have the highest calorie density, while watery foods with low fat content have the lowest calorie density. Calorie-dense foods need not be high in fat; most of the calories in a granola bar come from carbohydrate, but they're packed in a small space. Generally, the least calorie-dense diets contains more fruits and vegetables, less meat, fewer highly processed foods, more non- or low- calorie beverages and less total fat than a high calorie-density diet does.
... [A] little reduction in fat intake can go a long way.