Heavier is not warmer. It is one of the most stubborn misunderstandings about duvets. Warmth does not come from weight, but from the air the filling holds.
What a quality duvet does
A good duvet catches body warmth while letting enough air move. It forms an insulating layer without becoming heavy.
Duck and goose
Duck down usually needs more filling to give the same warmth as quality goose down. The result can be heavier and thicker, but not necessarily warmer.
The insulation value
The insulation value lies in the quality of the filling, the fill power and the finishing. A light high-quality duvet can feel warmer than a heavy lower-quality filling.
What you feel
Those sensitive to pressure or who get warm quickly notice this fast. A too heavy duvet can oppress. A nicely light duvet gives warmth without ballast.
Why heavy misleads
Weight feels convincing in the hand but says little about insulation. A heavy filling can mostly be mass. Quality down draws warmth from air, not from kilos.
What you better compare
Look at the ratio between filling, fill power and feel. A light duvet with nice volume can be warmer and more comfortable than a thicker one that falls flat or heavy.