I think the theory was that vegan food is less palatable and often less calorie dense and harder to find so vegans eat less. IIRC a study showed low carb diets

I think the theory was that vegan food is less palatable and often less calorie dense and harder to find so vegans eat less. IIRC a study showed low carb diets worked in a similar way, people just ate less as they found it harder to find enough calories.