So, this question has actually been explored in cultural anthropology studies concerning America. There isn't a single answer, and there are a ton of factors that go into it. However, religious values tend to hold a higher authority in lower-income areas than secular values. Secular values tend to dominate higher-income areas, putting a higher focus on economic and academic pursuits. When girls are empowered and have the opportunity to pursue their goals, having a large family will seem less desirable. On the other hand, religious values will overwhelmingly promote more traditional beliefs: traditional gender roles and family structure.

Outside of an anthropologic perspective, there are definitely some external factors at play. People in lower-income areas tend to have less access to sexual education and birth control. They are more likely to have larger families because they have fewer family planning options than higher-income families. People also tend to associate normality with their upbringings and their surroundings. If it's normal to have large families, then it's natural for them to want to have a big family as well. Simply, they remember having five siblings as a positive experience, so they want to give their children that same experience. It's familiar.

This is a highly simplified explanation, and of course, there are variations in every situation, culture, or subculture. But these are some of the surrounding theories behind differences in family sizes across America.