Why are so many vegetarians overweight?

Caloric density. Most vegetarian foods have fewer calories in a given amount of volume/weight than omnivorous foods. Fruits and vegetables have very few calories- a pound of brocolli has about 150 calories, blueberries perhaps 300. You'd need to consume almost a dozen pounds of certain foods to reach your daily caloric needs! On the otherhand meat, especially fatty meat like beef, has many,many more calories per pound. A pound of ground beef can have around 1500 calories! This isn't to say that vegetarianism is necessary or a guarantee for weight loss. Vegetarians could pig out on calorically-dense chocolate raisins and gain weight just like anyone else. Likewise, you don't need to be a complete vegetarian to be able to eat a greater mass of food without gaining additional weight. Simply choose more fruits and vegetables (without added sugar or oil) instead of sweets, whole grains instead of enriched wheat flour (the insoluble fiber in whole grains means there's fewer calories per pound, along with a host of other benefits) and get your protein from lean meats or soy that haven't been adulterated (deep-fried, creamed, battered, and what not)