It's a natural process when you get more weathy and more years of education. Before you can have a child and when the child was 12 years old they can share the family responsibility and make some income for the family, and they also can take care of younger siblings for the family, so the more children, the merrier.
Couples of years ago China implemented 15 years free but compulsory education for all children from kindergartan to high school in Xinjiang, and almost every teenager now also goes to college in this region, this means if you have a child, the child won't make any income and share any family responsibility until at least in their early 20's, this makes raising a child way more expensive, would you still like to have many children and can you afford doing so?