That is why vegetarians tend to be leaner than omnivores - but it’s still possible to overeat vegetarian food, or vegan food.
Indeed; one of the biggest guys I know is a vegan. Not obese, but about 6’8" and stocky. As best I can tell (I worked in the cube next to his), he spent almost every waking moment eating nuts. Most vegans/vegetarians I know are pretty slim, though. While it’s possible they have healthier lifestyles overall, I suspect the bigger factor is just that they eat fewer calories to satisfy their appetite.
And you don’t think there is a weight setpoint given that incredible consistency?
I remember reading somewhere that a person needs about four pounds of food a day to feel full. Four pounds of meat, dairy, or eggs is going to have a lot more calories than four pounds of most plant foods, though nuts are a notable exception. And, it’s fairly easy to gorge on processed foods. Did you know Oreos are vegan?
Of course, there are other factors in satiety; I’ve found that not eating enough fat leaves me ravenous, though not eating enough protein doesn’t have as much of an effect. So there are a lot of confounding factors. But yes, in general, people who eat less animal products tend to be thinner.
Well, no, because I wonder why I’m the exception among my friends and family 
. Even the ones at a healthy weight have to struggle with it, by going to the gym frequently and complaining about their meals.
For me, it’s like steering a bicycle in a straight line. Just requires gentle nudges left and right to keep on track. I weigh myself daily, mentally filter out the noise (about +/- 1.5 lbs), and if I find myself consistently on the high side of 125 I eat lighter meals for a while, and if on the low side I eat heavier meals for a bit. And I pretty much literally mean lighter–I don’t change the composition of my meals much, just the amount. I can see a response within just a few days.
But I have a few advantages compared to most. Like I said, my feeling of hunger is just extremely weak. I can’t know how others feel on the inside, but in terms of “food-seeking behavior”, I’d say it takes me about 36 hours to feel as hungry as most do after 6 hours. Another likely advantage is that I’m a slow eater, which presumably gives my body’s natural appetite suppression effects more time to kick in.
Finally, it seems that most people have some constant low-level hunger that drives them to snack. I hardly ever snack. I didn’t realize this difference for a while, but after going on several vacations with others where I spent a lot of time alongside them, it became clear that lots of people just snack constantly, to the point where they barely perceive it. It is just completely normal for them to always carry snacks or stop for something if it’s been more than a couple of hours since the last meal.
Do you drink, if you don’t mind my asking? I feel like that’s a big factor for a lot of people, along with the snacking. For some it probably drives a lot of the snacking.
Very little. Less than one a week, on average. You’re right; that’s probably a significant factor in many cases.