Even that overstates the case a bit. Remember your body needs about 2000 calories a day, just to live. Exercise adds caloric requirements above that level. So, you could eat 2-3 Big Macs a day, with no weight gain, if you did not eat anything else.
Walk 4-5 miles a day, and you can add a Big Mac *on top* of your ordinary diet, and not gain weight.
Most people do not realize that it is only a small amount of calories that make the difference in whether you gain or lose weight. Eat 100 calories a day less (half a can of soda pop) and you lose 10 lbs a year. Eat 100 calories a day more, and you gain 10 lbs a year.
Eat a little less, and exercise a little more, and most people can regulate their weight pretty close to where they want it. But, it takes will power and most people are not willing to make it high enough on their priority list.