There have been general assumptions that wealthy and urban people have small families.
This is true for the hard working affluent. People who are upper middle class but have two working parents.
Back in 2011, The New York Post wrote about the trend of large families in Manhattan, citing a study by the Council on Contemporary Families that had noted a “significant” uptick in families with three or four children among the top 2% of US households.
All over Manhattan, large families have become a status symbol. Four beautiful children named after kings and pieces of fruit are a way of saying, “I can afford a four-bedroom apartment and a hundred and fifty thousand dollars in elementary-school tuition fees each year. How you livin’?”
Steven Martin, professor of sociology at the University of Maryland, has actual numbers on the issue. “Families in the top 10 percent or even top 5 percent of household earnings aren’t having detectably larger families,” he wrote in 2008. But the story is different for Americans in the top 1 to 1.5 percent: “There has been a significant rise in the proportion of three- and four-child families among the super-rich.” In another analysis, he notes that the proportion of affluent American families with four or more kids increased from 7 percent in 1991-1996 to 11 percent in 1998-2004.
If nuclear fusion or advanced nuclear fission technology makes energy ten times cheaper and more abundant and advanced nanotechnology and other technology provides everyone with ten times more per capita income, then the future middle class could become like todays superrich. If carrying capacity of Earth and the solar system become a non-issue then there could be a new population boom among a mass affluent class.