One possible answer is that the average price per calorie in America is inverted from most of the rest of the world.
For example: A 79 cent, 81.0g apple with the skin on is 81 kcal whereas a 79 cent, 51.0g M&M Mars 3 Musketeers Bar is a whopping 212 kcal.
The key is that the apple is just over 1 cent per calorie while the candy bar is only 0.37 cents per calorie. Not to forget to mention calories by weight: 1 kcal per gram for the apple vs. over 4 kcal per gram for the candy bar.
In short, cheaper food has a higher caloric content on average than fresh foods yielding bigger asses.
(
nutritional information sourced from cyberdiet.com)