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.
So the down inside that 700 fill power The North Face Nuptse vest you have had hanging in your closet for years will fill 700 cubic inches for every 30 grams. A different type of material might only take up 550 cubic inches – i.e. 550 fill power.
How much loft a down insulation might be able to provide is based on the size and strength of the down cluster itself. The larger the down cluster, the more volume it will fill for equal weight. In some cases, stronger down clusters might also be able to hold larger air pockets for a slightly higher fill power than would be expected given the size of the cluster itself.
Size and strength - and thus warmth - is all determined through sourcing.
Geese are generally bigger birds and thus grow potentially larger clusters than duck. Duck also tend to be much younger when they are consumed and thus, smaller down clusters are collected.
Region and supply chain are also factors in 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 clusters.