Vegans are less likely than omnivores to be overweight or obese on average, which is a nice selling point for veganism. In general, our diets are somewhat lower in fat and much higher in fiber—two important factors for weight management.
But not everyone loses weight when they go vegan, and promising people that they will is unfair. There are plenty of vegans who struggle with their weight. Some overweight vegans feel guilty about their body size because they believe that they aren’t a good advertisement for veganism. But anyone who makes an effort to embrace habits that honor the rights and welfare of animals deserves to feel proud. Veganism is a lifestyle based on a set of beliefs about animal use and there is room for everyone to be a good role model.
That doesn’t mean that health is unimportant or that we shouldn’t strive to eat in a way that reduces risk for chronic disease. But, for whatever reason—and there is a lot about weight control that we don’t understand—some people, vegan or not, struggle more than others with weight.
There is plenty of research on how to lose weight and, for the most part, the formula is pretty straightforward: Move more to boost your calorie needs; eat less to decrease your calorie intake. Dietary approaches that allow you to “eat all you want without counting calories” nearly always achieve this by restricting food choices. But diets that restrict too many foods often don’t work well for long term maintenance.
If you’re struggling to lose or maintain weight, or find it difficult to stick with a reduced-calorie plan, consider tweaking your diet composition in ways that have been shown to improve success.
Find the right balance of fat—not too high and not too low. Evidence suggests that those who reduce their fat intake are more successful with long-term weight control. Because fat is energy dense, avoiding fatty foods can help reduce calorie intake. That’s why very low-fat diets—which sometimes ban all added fats and higher fat foods—are sometimes promoted as a foolproof way to lose weight. But while this is a very effective approach for some, it doesn’t work for everyone. Some dieters find they never feel quite satisfied without a few fattier foods on their menu, and so they end up overeating. Research suggests that including higher-fat foods in weight loss plans could be more effective for long term weight maintenance for this reason. (1) 
Certain higher-fat foods could have unique advantages as well. People who consume nuts (tree nuts and/or peanuts) tend to have a lower body weight than those who don’t. Nuts are associated with increased satiety and they may also cause a slight increase in energy expenditure. There is evidence that their fat calories are not well-absorbed, too. (2,3)  This doesn’t mean that adding big bowls of nuts to your diet will produce weight loss. But a serving or two of nuts per day (a serving is an ounce or ¼ cup) is not only compatible with weight loss efforts, but maybe even be helpful.
Load up on vegetables. The higher fiber intake of vegans is a real advantage for weight control since bulky plant foods create a feeling of fullness. Vegetables are especially valuable in this regard since they are rich in fiber and also high in water which gives them more volume. There isn’t much research on the relationship between vegetable consumption and weight control, but studies suggest that they are probably beneficial. (4)
Choose foods with a lower glycemic index. Carboydrate-rich foods with a low glycemic index (GI) are digested and absorbed more slowly and gradually which results in more gradual increases in blood glucose and insulin. One theory is that this delays hunger and helps with weight control. The studies are very conflicting, though. (5,6) It may be enough to just include plenty of fiber in your diet, but choosing foods with a lower GI can’t hurt. Fiber, fat, protein and acidic ingredients like lemon juice and vinegar all reduce the glycemic index of meals. Raw and gently cooked foods have a lower GI than well-cooked foods. “Particle size” affects GI, as well. For example whole wheat kernels have a lower GI than whole wheat flour. It’s the same food, but the GI goes up as the wheat is ground into smaller particles.
Give protein intake a boost. A little extra protein may help with hunger and protect muscle during weight loss. (7) Legumes are sort of the best of all worlds in this regard because they also provide lots of fiber. But other protein-rich foods like tofu and veggie meats can be useful, too.
Make time for both aerobic and strengthening exercise. Aerobic exercise burns calories and strength training protects your body from muscle loss during weight loss. Both are important for anyone who is working on losing body fat.
1.  McManus, K., Antinoro, L. & Sacks, F. (2001) A randomized controlled trial of a moderate-fat, low-energy diet compared with a low fat, low-energy diet for weight loss in overweight adults. Int J Obes Relat Metab Disord 25: 1503-1511.
2. Mattes, R. D., Kris-Etherton, P. M. & Foster, G. D. (2008) Impact of peanuts and tree nuts on body weight and healthy weight loss in adults. J Nutr 138: 1741S-1745S.
3.  Alper, C. M. & Mattes, R. D. (2002) Effects of chronic peanut consumption on energy balance and hedonics. Int J Obes Relat Metab Disord 26: 1129-1137.
4.  Rolls, B. J., Ello-Martin, J. A. & Tohill, B. C. (2004) What can intervention studies tell us about the relationship between fruit and vegetable consumption and weight management? Nutr Rev 62: 1-17.
5.  Abete, I., Parra, D. & Martinez, J. A. (2008) Energy-restricted diets based on a distinct food selection affecting the glycemic index induce different weight loss and oxidative response. Clin Nutr 27: 545-551.
6.  Das, S. K., Gilhooly, C. H., Golden, J. K., et al.  (2007) Long-term effects of 2 energy-restricted diets differing in glycemic load on dietary adherence, body composition, and metabolism in CALERIE: a 1-y randomized controlled trial. Am J Clin Nutr 85: 1023-1030.
7.  Layman, D. K., Boileau, R. A., Erickson, D. J., et al. (2003) A reduced ratio of dietary carbohydrate to protein improves body composition and blood lipid profiles during weight loss in adult women. J Nutr 133: 411-417.