Silly question. The amount of food (particularly fat-filled garbage) versus the amount of exercise (and, playing video games doesn't count!).
There's a lot in this area we don't know, and much in which the scientific studies are contradictory. But one thing is absolutely clear:
Obesity is not a simple matter of calories consumed vs. calories expended. People don't become obese because they eat too much and exercise too little. The metabolic disorders that cause obesity also cause lethargy and hunger.
(And there's nothing unhealthy about natural fat.)
I'm not sure that's true. Look at Asians and Africans as an example. It's rare to see an obese Asian, whether in Asia or in America. Most of them eschew the garbage diet that government meddlers and left-field "nutritionists" have cooked up for us. The same is true of Africans. Most Africans are thin, largely due to a diet that may depend on the available food supply and an ability to obtain any of it.
However, migrate to America and what happens? A vast majority of African-Americans become obese because they indulge in the crappy American diet of fast food, pizza and chips. However, Asians in America still tend to remain slim and healthy, largely because they retain the traditional Asian diet.
Finally, we have been inundated with "reserach" and "studies" that indicate that this or that is bad for us, only to be disproved by the next "research" or "study" that says "no, it isn't"! So, what are we supposed to believe? At the end of the day, I land on the side of moderation in everything. No fast food every day, no bags of chips and dip and sitting on my butt for hours at a time watching TV or working at a desk. If I need to talk to someone two doors over or even 3 floor up, I walk to their desk and give my phone a rest.
As a nation, we have become lazy and spoiled and accustomed to a diet of crap, garbage and massive calories, then we listen to idiots (like the FLOTUS) who tells us what we should eat (not that SHE keeps the same diet the rest of us are supposed to).
Way back in the olden days, schools taught courses like "Home Ec" that helped young women learn about nutrition and cooking/serving healthy foods. After the women's lib movement emerged in the late 60s, such courses were deemed "sexist" and were dropped from school curriculums. So, it's no wonder that we are a nation of obese people. With all the mixed messages and too many people choosing to eat out instead of preparing meals at home and fewer of us exercising outside, it's not a surprise we are a nation of obese people.