Not necessarily. A vegetarian diet is not inherently a weight-loss diet, but rather a lifestyle choice.
It is true, however, that adults and children who follow a vegetarian diet are generally leaner than those who follow a nonvegetarian diet. This may be because a vegetarian diet typically emphasizes more fruits and vegetables and includes whole grains and plant-based proteins — foods that are more filling, less calorie dense and lower in fat.
But a vegetarian diet isn't automatically low calorie. You can gain weight on a vegetarian diet if your portion sizes are too big or if you eat too many high-calorie foods, such as sweetened beverages, fried items, snack foods and desserts.
Even some foods marketed as vegetarian can be high in calories and fat, such as soy hot dogs, soy cheese, refried beans and snack bars.
Whether you avoid animal products or not, achieving and maintaining a healthy weight comes down to eating a healthy diet and balancing calories eaten with calories burned.