I think that depending on the individual's diet, and what they ate before becoming vegan, people may tend to lose weight. Unfortunately, it is still easy to eat a high-fat vegetarian or vegan diet if you are not careful and avoid treat foods and fried foods. I think that the "full" feeling is usually due to the higher fibre content in vegetables and whole grain foods. This also helps to keep people from overeating as you feel full faster and it takes longer to eat (more chewing). I also think that once a person develops healthy eating habits, junk foods lose their appeal, and can even cause stomach upset.
Here is a link that might be useful: Vegetarian-Tips