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.
I'd argue it's basically entirely this. The modern food environment is incredible (over)-stimulating. Food is ubiquitous and abundant, with a dizzying array of novel variations. Food cues enticing you to eat are ever present.
Being vegan has the accidental side effect of counteracting much of this. A lot of the built-up food infrastructure suddenly becomes non-existent.
Imagine. You're walking into a busy mall in a foreign country. On your left, there's a café advertising a variety of tempting pastries. Your sister told you about that cafe. She loves their cream cheese cake. To your right, the sizzling flames from a steakhouse grab your attention. Moving to another level, you spot an ice cream parlor showcasing a new flavor: Strawberry Daifuku. But all this has to wait, because you're heading straight for the ramen shop you've heard so much about. They're known for their amazing pork bone broth. It's why your foodie friends have chosen to meet up here.
Now if you were vegan, all of that is just ... gone. Possibly the foodie friends as well ;)
Non-vegans don't think about this but they recognize it subconsciously -- most non-vegans would balk at having to go vegan even for a short time, just due to the inconvenience alone.
Some of the aspects of the modern food environment come back when you figure out where to look of course, but even the even a savvy vegan consumer won't encounter the same level of choice, variety and catering and let's not forget cost-competitiveness that a modern omnivore experiences.
The restaurant scene seems to be a lot better in Canada. Most restaurants have vegan options. My work's Christmas lunch was at a steakhouse of all places, and I still had several options to choose from.
Most of what I refrain from eating is the junk food that seems to suffuse work and social life: donuts, cookies, and cake leftover from one meeting or another, and buttery/cheesy/eggy snacks at social gathering.
I didn't claim anything counter to that. I can cook plenty of tasty vegan food. But if I go out there's not much left in my country to eat. You have to go to a handful of specialized vegan restaurants in the city to have even somewhat a choice. Otherwise it's fries or noodles with tomato sauce, which personally I wouldn't consider a meal.
That it's better in Canada? Great. I believe you, but I've travelled a lot, and I haven't seen many places that have such a variety of vegan food as USA and Canada.
Most of Europe is very meat heavy, vegetarian is still easy, vegan is picky eating. In East Asia there are vegan options, but often tasteless, because they're meant for Buddhist monks. Otherwise even vegetarian is somewhat difficult.
In India vegetarian is easier than meat, especially considering food safety. Had 0 gastric issues with vegetarian food, while one bad piece of meat almost ended the trip for a friend of mine. But a lot of dishes have butter and milk, so vegan again is difficult.
I'm really not saying it's difficult to cook great vegan food, just that it limits your ability to eat outside in most of the world, which probably influences average BMI more than the diet itself.
It's also likely that they simply eat less because vegan food just isn't very good.
I think this is mainly as a response to people using plant based fats for deep frying. Traditionally they were fried in beef tallow, no need for any flavour enhancers if you fry them in beef fat.
Also, fries need mayo, which is made with eggs.
Most people would think these foods boring, but that's because they've only ever had the shitty, mass-produced versions.
Add some fried bacon bits and tell me it's not infinitely better for it.
> fresh rustic sourdough bread
Which is great on its own, but much better with some butter and ham.
> vegetables roasted until they caramelize
Which is a great side dish next to a good steak.
Nothing wrong with vegetables, but none of these things are as satisfying as they would be combined with some animal products.
No evidence of this, but I also suspect that vegans care more about their health in other weights too (e.g. exercising, not smoking, etc). If they can keep up with a vegan diet, they likely also have the discipline to keep up with exercise regimes and keeping good habits.