A lot of people consider the high fat diet as the causes of the weight gain. However, some high fat diets contain healthy fats. Moreover, the lean protein, fruits, veggies and whole grains can actually allow you to lose weight. Weight gain happens when the food you eat has more calories than your body is burning. So it depends on the kind of fat and total calories. Fat comes packages with higher calories per gram than protein. Eating a high-fat diet may or may not cause weight gain than a high-protein diet.