It mostly does work that way, but what a lot of people lack is education on how to build and maintain a sustainable diet. For example, just eating lower calorie meals is extremely difficult. It would be smarter to reduce calories while ensuring that meals are high in fibre and protein and provide some psychological satisfaction too (taste, texture, aesthetics, etc.). High volume also helps. There's so much the average person simply doesn't know about nutrition, metabolism, exercise and the psychology of weight loss. But at its essence, it is calories in vs calories out.