Thin people favor bulky foods.

Barbara Rolls, a professor of nutrition at Pennsylvania State University, has done extensive research on “calorie density,” or the ratio of calories to the weight of food.

Simply put, foods with a high water content — fruits, vegetables, water-based soups and stews, and cooked whole grains — are low in calories but satiating. Most also contain lots of fiber (an apple has three grams; one cup of cooked barley has six), which fills you up.

Whether consciously or not, many thin people follow the strategy of starting out with a sizable soup or salad, which leads them to eat less for the rest of the meal. One Rolls-led study found that subjects who began a meal with a low-calorie salad — about 100 calories for three cups — were more likely to eat fewer total calories. “It subtracted about 12 percent of the calories from the meal,” she says. Foods with a lot of water, she adds, “can help you perceive that you’ve eaten more.” Drinking water with a meal, Rolls has found, doesn’t have the same effect.