It is really a simple matter of calories in vs calories out.
Unfortunately, it seems like it would be that simple but it isn't. Certainly, you are correct in that regardless of what kind of diet you do, you can't eat 5,000 calories a day and expect to drop weight, unless you're Michael Phelps. But something like a keto diet sets up a metabolic advantage in your body that will allow someone to lose more fat than other diets of similar daily calories.
Shifting dramatically towards protein and fat and away from carbohydrates does three things that make a significant difference. First off, the body significantly increases the production of fat consuming enzymes, for processing dietary fat but also stored body fat. Second, there's a significant reduction in the production of insulin, which is the body's fat storage hormone. Lastly, a keto type diet creates more of a muscle sparring environment in the body so more of the weight loss is fat vs muscle. And retaining muscle keeps your metabolism from tanking.
