The extreme warmth-to-weight ratio that can be achieved by down is due to the size of insulating air pockets created. This is referred to as loft, and measured through the often confusing denomination of Fill Power.
In the end, Fill Power is simply a measurement of the volume created by a given amount of down – almost always measured in 30 grams per cubic inch.
Geese are generally bigger birds and thus grow potentially larger clusters than ducks. Ducks also tend to be much younger when they are consumed for their meat, and thus, smaller down clusters are collected.
Region and supply chain are also factors determining warmth.
Those birds from collector-based supply chains raised in colder parts of the world will be older before consumed, offering larger clusters, and the colder environments can produce stronger (and therefore warmer) clusters.