A mixture of factors are behind this, ranging from policymaking to economic growth to more migrants, she added.

Data from the United Nations showed that family size ranges from two to nine persons globally. Smaller families are commonly found in wealthier regions such as Europe and North America, rather than in developing countries.

Shortly after the census data was released early last month, Ning Jizhe, director of the National Bureau of Statistics, offered an explanation.

He told a news conference in Beijing that increased migration and improved housing were among the major contributors.

"The young move out when married," he said, adding that the flagging fertility rate and more divorcees have also advanced the "miniaturization" of Chinese families.

The NBS data showed that the per capita residential area for urban and rural residents reached 39 and 47 square meters in 2018, up from 32 and 39 square meters from 1978, respectively.

Yang Yifan, a demographer in Chengdu, Sichuan province, said China's recent economic boom, its world-class digital infrastructure and its unique parent-child bonding have together made moving out a preferred choice, and increasingly a social norm.

On one hand, better educated Chinese seek independence by living apart from older generations due to diverging habits and values, and on the other, they have maintained rather close ties through frequent video calls or mutual visits made possible by the sprawling high-speed railway network, he said.

"It is a parental obligation in China to help children buy homes with their lifelong savings," said Yang, deputy head of the National Interdisciplinary Institute on Aging at Southwest Jiaotong University.

"That has made it financially possible to create new and smaller families."