People that claim "it's a glandular problem" or "I have big bones" are really eating more than they let on or even know themselves. The next time someone says "I don't know why I gain weight, I don't that much at all", watch them all day and see what they eat. Or get them to write down what they eat and how much of it they ate. It's a real eye opener.
People that go on fad diets, lose some weight, then go off the diet and gain the weight back are training their bodies to be more efficient in storing the food as fat. There are people that gain weight while eating only 1000 calories a day.
By having a daily exercise routine, you train your body to be on "alert" to burn calories and keep the fires at the ready for converting more fat and carbs to energy. The body doesn't pack everything away as fat because it knows it'll be needing energy soon and that's why active people are more alert, cheerful, think better, and in general, have that annoying healthy glow and upbeat attitude that really irritates the sloths who are still trying to wake up with that third cup of coffee.