I'm not aware of them having significantly more kids than anyone else
It's pretty well documented.
For example, in 2010 the annual fertility rate for those with annual household incomes under $10K is about 99 (per 1000 women). For households with about $30K annual income the fertility rate is about 67. For households with about $60K the fertility rate is about 54.
It also tracks somewhat with GDP by country. There are a few exceptions of course. Here's a plot of a different sort of fertility rate (total children per adult woman) versus GDP:
If you put that on a semilog plot with raw data on the vertical axis and ln(GDP) on the horizontal axis, it is easier to see the downward slope.
There are a number of scholarly articles out there attempting to explain this phenomenon. A google search will turn up too many. I prefer my own rather low-brow analysis.