A counterpoint: some of the biggest and best advances in the developed world occurred during or after a time of high fertility. The boomer generation is a good example. I'm not a boomer (I'm < 30), they get a lot of hate, but that generation basically invented the entirety of the modern world, without which we wouldn't even have the tools to understand climate change. If you want to achieve great things you need people to exist and do the work.

Examples of high fertility rates combined with poverty are often correlated with dictatorships and corruption, in my opinion they are cultural.