It is not the down clusters alone that keep you warm. It is mostly the air they hold. Between millions of fine branches a layer forms that holds your body warmth.
Where the down sits
Goose down grows close to the body of the goose, hidden under the feathers. Each cluster has fine branches that trap air. That air forms a soft buffer between you and the room.
Larger clusters, less filling
Larger down clusters carry more air. So you need less filling for the same insulation. The duvet stays lighter, while still being warm.
The misunderstanding
A heavier duvet is not automatically warmer. Weight can even oppress. The insulation value lies in the quality of the filling and the air it can carry.
What fits you
Those who get cold quickly look for a good insulation layer. Those who get warm quickly look for lightness and ventilation. Both ask not for another principle, but for another balance.
The misunderstanding of warmth
A duvet does not produce warmth like a hot-water bottle. It keeps your own body warmth in trapped air. That explains why a light duvet can still be warm.
Balance in two directions
A good duvet not only holds warmth. It must also be able to breathe when you get too warm. Comfort arises precisely in that balance: protecting without locking in.
A last nuance
As often with good sleep, it is not about one single detail. Material, size, care and feel work together. Only when that whole fits does comfort become natural, and you no longer need to think about it at night.