It's extraordinary how often otherwise civilized people think it's not only their right but their duty to criticize the size of other people's families. I freely confess to doing so myself on occasion, though I do try to limit my comments to general cases, not specific people. Maybe it's because the only remaining area of our sex lives where criticism has not been taken off the table is its fruit (or lack thereof).
Most annoying are the self-righteous critics. You know, the ones who insist that sweet little baby you just gave birth to will destroy the ecological balance of the world. Or those who praise God for the gift of antibiotics and other life-changing interventions while solemnly intoning that your use of birth control betrays your basic lack of trust in God's plan for your family. There are valid points lurking behind both of those extremes, but there is room for such a wide range of disagreement that prudence and courtesy—not to mention the love we owe our fellow human beings, and the good ol' Golden Rule—call us to admit that the size of other people's families is no one's business but their own.
That said, I recently found a Front Porch Republic article that explicates one of the negative side effects of the recent trend toward small families. I highly recommend reading the entire article, but will quote here as much as I think I can without raising the ire of the copyright fairies.
China’s one-child policy has stripped the social space between the state and the individual of every protection that the most natural community, the family, can provide. How much more damaging must be the collapse of family size in Chinese culture, in which family ties have played such an important role? I know I’m not saying anything new ... but we cannot remind ourselves too often of what is lost as family sizes have collapsed (through state coercion or, more sadly, voluntarily).
As American family sizes shrink we get a glimpse of the losses, but we may not be aware of the extent of the social damage caused by a one-child policy (or the one-child norm that some would foist upon us). ... If we assume that everyone gets married and has the same number of children—big assumptions I know, but they give a sense of the social stakes in family size—the table below gives the number of relatives (up to first cousins) in a steady state:
We don’t need a chart to see that a reduction in family size reduces the number of siblings, but the fact that the number of parents and grandparents is stable may distract us from the collapse in the number of aunts/uncles and cousins.
A family tree with many branches functions as a broad social safety net: when average family size falls from three to two there are only half as many aunts and uncles to lean on, to visit, to identify with, to support you when things go wrong and rejoice with you when things go right. When average family size is one there is little family left to protect you and to belong to. The modern fantasy—society as disconnected individuals under a tutelary state—becomes grimly plausible.
The family is most local of all communities, and its decline is at least as great a social calamity as the commercialization of culture and the state organization of society. ... You are not obligated to have as many children as you physically can, of course. Don’t rely on the culture’s norms in making decisions about family size, though: the culture will confirm your evaluation of the costs of more children, but it is blind to the great gift you are giving to your children and your descendants.