How many progressive-liberal-atheistic-or-agnostic-leftists in the US have six or more children? A conservative [pun intended] estimate? None. Not one.
How many old-fashioned-conservative-religious-people in the US have six or more children? LOADS!! Go to Williamsburg or Monsey or New Square and find me a family that DOESN'T have six or more children by choice. Li-havdil, Mormons and Catholics and other devout Christians also have large families. [Mormons have an advantage because with 5 plus wives - the possibilities are almost limitless!!].
Why is this? Do progressive-liberal-atheistic-or-agnostic-leftists not engage in sexual relations? Like, they only do it twice in their lifetime so that is why they only have 2 [or less - maybe 3] children? Not quite. They don't have large families because they make sure to take measures to avoid having children. WHY? Let nature run its course and hold a sweet new baby in your arms whom you can love!! What is greater than the love a parent has for a child?!!
Here is the crux of the issue. For a religious person, a child is a blessing from Hashem. A gift. The child will possibly get in the way of advancing my career [if I am a woman], self-fulfillment, traveling freely and a good night's sleep. Children are EXTREMELY expensive. I could spend that same money on MYSELF!! Why waste it on someone else? None of those factors outweigh the incredible responsibility we have to bring as many children as we can into the world and teach them to be sweet, holy, G-dly human beings. A person is created in the image of G-d. The more people - the more G-dliness.
The secular person doesn't see it this way at all. They will have children [usually but many many people opt out completely and instead raise cats and/or dogs. Even when it is not raining] but that is because of their NATURAL, INBORN desire to be a parent. Not out of a sense of obligation or responsibility. And this natural desire is fulfilled with one to three children and that is it.
As an example, take the 83 people running for the Democratic nomination for President. Some have no children at all [especially the guy married to the guy...]. Some have one or two. I doubt if any of them has four or more. And it is not a matter of money. They all had good jobs and earned well in their lives. It is a philosophy of life. [When they talk about "reproductive rights" it always means the rights NOT to have a child. Halacha talks about "reproductive obligations" to have children!]
I, for one am thankful for the gift of 6 children and HA-LEVAI that it would have been 16. I am also happy that my parents didn't stop after 2. If they had - you wouldn't be reading this and I wouldn't be writing this. Think of all the people you know who are 3rd, 4th, 5th, 6th born children. Wouldn't the world be poorer without them??