China and India are home to some of the oldest civilisations on this planet. Their relatively large areas combined with the fact that both countries boast sizable sections suitable for large-scale agriculture where crops can be grown for most of the year as opposed to a single season led to a need for a great number of people to farm that land. And because traditionally most farms were small and family-operated, the more children one had, the more hands there were to do the work. And as those societies developed, a greater number of offspring meant more sons to send to work and more daughters to marry off, which in turn led to sons being more desirable in both places. Few other countries that size have the resources and climate to provide for that number of people, and none has a historical population anywhere near that size anyway. COULD Russia, Canada, the US, Australia and Brazil each support a billion people? Yes, with modern agricultural methods. But why would they even try?
The combined population of Russia, Canada, the US, Australia and Brazil probably wouldn't exceed 600 million, yet their combined area would be nearly 5 times that of China and India put together.