I was reading about Mozart, and he had 6 siblings but only 1 of them survived infancy. I think this was recurrent back in the day as they didn't have treatment for diseases, basic sanitation, etc...
But in the 20th century, as medicine, technology started to evolve and meds, vaccines, sanitation, etc.. started to improve, children were not dying as before, but people kept having many children and population exploded.
I read that many scientists and knowlegeable people back in the day though that the more comfortable life became, the less children people would have. But this didn't happen as women kept having 8, 10 children as before, they wouldn't die and they themselves would have more 5,6 children and so on... leading to the huge population we have today.