Larger creatures, and those higher on the food chain, having greater access to resources and higher survival rates, invest more in fewer children. Smaller creatures, with lesser access to resources and higher mortality rates, invest less in more children.

A rabbit doe can theoretically have 480 offspring, at 12 per litter, 10 times per year, over 4 years. That's the kind of thing you might do if 98% of your kids get eaten, because you will still have 10 left. Rabbits can also abort their litters when stressed.

An elephant might have one calf every four years, over 55 years, or a maximum of 14 children. That's what you might do if only one other species can reliably kill any of you, and you expect all of your babies to survive to adulthood.

With humans, the observed historical behavior is that poor or uneducated humans are relatively fecund, producing 8-30 children per pair, occasionally with multiple births. As wealth and education increase, and particularly when child mortality drops and elder care becomes socialized, birth rate drops down to the population replacement rate (or below). Middle-class first-worlders usually have 0-4 kids.