Through a random series of events during fall break (too confusing to detail here), my daughter spent the week with my in laws, and my son spent the week with my husband and me. Musing more on large families, I did notice the difference between caring for one versus two.
Some of the differences were obvious and simple mathematics. There was less laundry. I only had to wake up one child instead of two children. I had to follow up with one child about eating their vegetables instead of two.
Others were not so obvious. I had more time to talk with my son. With two children, the conversation is often hectic. I'll talk with one, but the other will comment or respond (or bring up something completely different). My son, on the whole, had a good week. He missed his sister a bit, but not so much that he was inconsolable. My daughter had a good time on her own as well.
So my theory, from this experience and others, is that each child adds an exponential amount of work for parents. It's not as simple as buying in bulk. I agree with chanson that parents have finite time and energy.
That's not to say that all parents should have one child to devote all their time, energy and attention to. If they do, that's great. As long as the only child learns to interact with their peers, to develop and maintain relationships and doesn't believe they are the center of the universe. (All of these are great goals for any children, regardless of siblings or lack thereof).
Being other focused, and taking other people's feeling and opinions into account are important skills. Also, being responsible and self-reliant, in other words, not falling into snowflake syndrome.
Because I have two children, I have the opportunity to do things with my kids that my parents were not able to do with six. There are many reasons for that, my parents' temperament and the nature of my family growing up. Things like going to pre-school are possible.
And it's certainly not a competition between my parenting and that of my parents. But it simply makes sense that with two children, a parent has the financial and physical resources to do more than they would with six. Will those additional opportunities benefit my kids? I have no idea, I certainly hope so.
My daughter is back home now; we are back to wrangling two six year olds and keeping up with non linear conversations. My feelings about the week were a bit like my feelings when we separated my twins into different cribs when they were four months old. They had slept in the same crib before that time. I was sad, and concerned that it would impact my kids. Both my son and daughter thrived in separate cribs, however. They slept much better without a sibling moving around and waking their other up.
Sometimes I think the milestones or significance is more about me than it is about them.