Eating fat doesn't directly make you fat - total calories matter more than fat content for weight gain.
High-quality evidence from large randomized controlled trials shows that reducing dietary fat intake leads to modest weight loss (about 1.4 kg on average), but this effect is primarily due to reduced total calorie intake rather than fat being uniquely fattening. Studies comparing diets with different fat percentages (20% vs 40% of calories) found similar weight loss when total calories were controlled, indicating that total energy intake - not fat intake specifically - drives weight gain.