Vegan food is incredibly more caloric dense and nutrient rich than meats. That's why high protein diets are used to lose weight, low carb diets were born because of this, meat has few calories and fills you up.
The Effects of Increased Protein Intake on Fullness: A Meta-Analysis and Its Limitations
It is easier to gain weight with a whole food vegan diet because you are restricting yourself to eating high caloric and nutrient rich foods instead of wasting stomach space with useless meat.
- Meat(muscle) has generally around 250-270 kilocalories per 100 grams
- Liver has around 165 kilocalories or less per 100 grams
- Eggs have 155 kilocalories per 100 grams, most of which is pure fat.
- Grains have generally around 330-375 kilocalories per 100 grams
- Beans(dry weight) have generally 330-480 kilocalories per 100 grams
- Nuts have generally 560-680 kilocalories per 100 grams
- Oils have generally 800-950 kilocaries per 100 grams
Source: Nutritional value of foods written on packages
The only caloric rich tissue present in animals is fat, but nobody eats pure lard in their diet in the modern day.
Those are the most consumed foods in the entire history of humanity, just look up the mediterranean diet, literally based on olive oil, grains and legumes... or any asian diet.
For example my first two meals for today were : 204 grams of muesli (990 kcals) + 250 grams(98 kcals), then just a few minutes ago 150 grams of pasta, by dry weight (534 kcals) + 100 grams of bread (280 kcals).
In only 2 meals I consumed 1930 kcals, not even counting the sauce from the cooked pasta, which is a lot because just a spoon of olive oil sits at around 124 kcals. Every time you eat meat you are consuming less calories than you could if you simply ate some vegan food instead.
But what about veggies? well, the most consumed foods in the world are grains not veggies for a reason.