Comfort under a down duvet is not a matter of thickness or weight. It is a matter of how well the duvet moves with your body.
Light
Natural down has three qualities that together make the difference. First, it is light. Because a large cluster of down weighs little, the duvet does not press upon your body. You feel warmth without pressure.
Breathing
Second, it breathes. When your body temperature rises at night, the filling takes that warmth in. When you cool, it gives the warmth back. You need not search for the right balance, it adjusts itself.
Adaptive
Third, it adapts. A natural down duvet folds with your movement. It closes along your body. A synthetic duvet stays taut, local, and leaves cold bridges everywhere.
Comfort under down is not something you notice. It is something you observe in the morning: you slept through.