Vegans still eat carbohydrates, carbohydrates are generally high in calories, and that's what makes them fat. Eating a lot of pasta, cookies, white rice, crackers, chips, anything with added sugar will make you explode. Our body processes carbs first, but is also very happy to store those excess carbs away as fat. Eating a lot of carbs will also make your blood sugar go up and down more than it should, and you will end up eating more because having your blood sugar plummet will make you hungry. Protein doesn't get stored the way fat does, and it also helps make you feel full, making it easier to keep your weight down. Eating fats like butter, cheese, yogurt, etc actually helps with weight loss because it gives a lot of energy and also makes you feel full. Veggies you could eat in excess until you're full, since most veggies are low calorie, and generally have a lot of nutrients.