But Charba it only matters if population growth is an issue.
I think the worry with many DC is whats requires of the earth to support two people, or one, being facilitated through unsustainable means.
As in, one must have a range rover, have CH on to over 18° constantly, continually run tumbledriers, must fly all DC to remote holiday destinations, can only be supported by having to fly or drive long distances for work to support this many DC, and that it's contributing to further pop'n growth.
Much of what living in a large family group is about is economies of scale, but for many its only because of unsustainable means of achieving that.
As I recently saw on another thread about earth's sustainability, there are large families in Africa for instance, that make less eco damage than a childless individual in the UK.