They have jobs that don't result in being so wiped out that evenings are spent lounging in front of the tv.
Thin people are often veteran fridge raiders.
Fridge raiding, in moderate quantities, dissolves the distraction of an empty tummy usually within 10-15 minutes of eating something. Fat and protein, most especially fat, are hard for the body to process. The result is that your metabolic rate will go through the roof, but only if your carbohydrate consumption is very small. So by eating lots of fat and protein you'll start burning calories without excersize and reduce the amount you eat at the same time. See this excellent article in The Daily Telegraph , by Nigella Lawson, on the subject of high-fat diets.