Some of the reasons you listed are considerations for us, but they are not the main reason so I voted other. Like Embee said, we will have only one or two children (still thinking about whether to have another) because we like the family size. It's cozy and intimate and fun. It's how we always pictured being a family.
Some of it does have to do with our temperaments. Lots of people drain, not energize, both of us so I think that's a big part of why we never pictured something big. Also, it just isn't my personality to be momma to a big clan. Don't know how else to say it. It's not who I am. And, if we have another, the two will be four or more years apart for the "enjoy each babyhood" reason.
The environment is a consideration and always has been, but I think if I were one who thrived on the energy of lots of kids and projects and chaos underfoot, I would find a way to explain it away. As it is, it does feel comfortable to say we will leave no more than we are.