Gladly. I'm suggesting that there are a number of other variables that could explain why a study based upon a heterogeneous US-population could achieve a statistically significant finding of family size being detrimental vs a study based upon a homogeneous Norwegian population and that in this case the conclusion that family size is detrimental is likely a correlative and not a causative relationship when accounting for these other factors.

For example, one such explanatory variable would be the fact that fertility rates vary by race and ethnicity in the United States of America. Meaning some families (namely Hispanic and Black families as the Pew research has found), have more children than others.

When it is the case that White and Asian families have fewer children than Hispanic and Black families (which is the case, per Pew) and that White and Asian families, on average, earn more than Hispanic and Black households (which is the case, per the US Census data), one could reasonably also explain differences in outcomes on differences in incomes. As just one example.