Many people think a heavier duvet is warmer. With good down that is not so.
The logic
Warmth under a down duvet does not come from the weight of the filling. It comes from the air the filling can hold. A good down duvet weighs light and holds much warmth. A lesser alternative weighs heavier and holds less.
The criterion that matters is the ratio between fill power and weight. The higher the fill power, the fewer grams of filling you need for the same warmth. And the less the duvet weighs, the less it presses upon your body.
In practice
In practice: a good winter duvet weighs about 850 to 1100 grams of filling, depending on size. A synthetic alternative at the same warmth grade often weighs double.
Whoever suffers from a heavy feeling at night, or wakes with stiff shoulders, often asks whether the mattress is the problem. Sometimes it is. But just as often it lies with a duvet too heavy that never did its work well.