I think it's more to do with lack of wars wiping people out, fatal diseases being eradicated and better healthcare meaning people live longer (as babies are born a disproportionate number of people die) and less babies die when just a few decades ago in some countries you would be lucky if half of your **** lived past 3. Then in general society you have more teenage pregnancies than ever before and people getting married multiple times (ie no marriage for life with one partner anymore) and having ******** with each new partner.
It's insurance
when you live in an impoverished country with no pensions or healthcare it's prudent to breed a lot so you've got people to take care of you