If you’re not gaining weight, then you’re not in a caloric surplus. You may just have a limited appetite, but if you take in more calories then you expel, you will gain weight. It’s a very simple equation. Individual differences surrounding metabolic rate, hormonal function, calorie expenditure, and digestion will affect one’s ability to be in a surplus. Just filling up your stomach until you’re full isn’t a strategic enough plan to put on weight, especially if the goal is to build lean tissue. Taking in the right balance of protein, carbs and fats and at times that promote good digestion/assimilation is the strategic part of this. Providing a strong enough stimulus to initiate muscle growth is the first step, of course.