Not just care about health, but they're on a very restrictive diet.

Many of my vegan friends can barely find something to eat when we're going out. As an omnivore you can eat anywhere, as a vegetarian, you can eat almost anywhere. As a vegan, at least in Germany, it's still somewhat restrictive.

If you're consistently refraining from eating, then you're likely to just consume a lot less calories.

Another factor here is that most restaurants cook for attractiveness, not for health. Adding copious amounts of butter, oil and sugar can make a lot of food taste better at the expense of a much higher caloric intake.

I have an overweight vegan friend. He eats fries everywhere. Fat and carbs makes vegans fat, too.