This is an issue that is between God and each couple. No one has a right to speculate or judge or condemn anyone for the decision that is made between them and the Lord.
There are plenty of grave reasons why couples do not have larger families. It's amazing how many couples honestly can't have more children, even though they are open to life. I know several young couples who have been unsuccessful, and are consulting with the Pope Paul Institute to figure out their fertility. I agree that late marriage may have something to do with it. People have a lot of misconceptions about fertility, assuming that all people are fertile. Schools do not teach proper biology about fertility, and popular media perpetuates a lot of the incorrect teachings by stating that women can get pregnant at any time of the month.
I think that people with large families have to be careful not to become prideful or boastful. They have been blessed by God. He gives a man and a woman the ability to have children.
I attend a large, rather modern Catholic parish, and there are plenty of families with 4 or more children, and quite a few with 8 or more. So this is not a "traditional" Catholic thing--modern Catholics also have large families. We also have several families in our parish who have adopted large families.
I converted from evangelical Protestantism, and it is definitely becoming the norm for evangelical Protestants to have large families. The pastor of our church and his wife had 10 children (two were deceased). The associate pastor had 7 children. The leading family (money, time, church offices held) had 8 children. My best friend in the church had 6 children. There were other families in the church that I didn't know personally who had large families. In evangelical magazines and denominational publications, there have been several key articles about the value of children and why Christians need to consider having large families. So it's not just a "Catholic" thing, either.
One thing that no one has mentioned so far is the cost of having children these days. I agree that children do not need to grow up figure skating or attending a private prep school, which is what my two children had. I have absolutely no problem with families who give their children the bare basics of life (food, shelter, warmth, 2nd hand clothing, home-based recreational opportunities, only the basic health care that is provided by the health department, etc.)
But many couples, due to the state of our economy, or bad educational choices on their part, or just bad luck in the job market, are unable to even provide the basics to themselves, let alone children.
I have a problem when couples continue to have children, but cannot pay the health-care costs for the pregnancies and deliveries, and cannot provide the basics for the children, but instead, feed, house, and provide health care to those children through long-term government aid. I do not believe that these people are justified to continue to have biological children. I'm not sure that this is God's plan--to have children and force others to care for them.
I think that most of the people in the U.S. are quite hostile toward this practice, and IMO, this could pose a risk to these families and their children. I believe that such couples could lose their children, as the government that they have come to rely on can turn on them and take their children away and place them in foster homes that can provide the basic needs. I would support this practice, as I hate to see children who are suffering due to lack of basic food, warmth, clothing, shelter, or health care.
I think that we will eventually see laws that require sterilization for couples who continue to have "welfare babies." Yes, forced sterilization is immoral, but forcing others to pay for your children to have decent care is also wrong.
If such a couple has a church or parish that will willingly help them to provide for their large family, then that's OK. But many parishes balk at providing for children of parents who are unemployable, unless the unemployment has been caused by hardship such as injury, on-going illness (such as cancer), weather disasters (tornado, flood, etc.), or some other tragedy. If a couple has made poor choices earlier in life that led to their unemployability, then I believe a parish should shut the purse and tell the couple to wait to have children until they are in a place where they can care for them without aid. A parish would do well to provide such a couple with educational resources and scholarships for them to attend trade school, community college, or enter an apprenticeship.