Cultivators' small world gives birth to lives because their small world follows the rules of Heavens. It can also be said that their small world is actually not theirs but a part of the Heavens.
Take Holy Beast Continent for an example. It was originally the small world of Xiao Family's founder as well as Su Xue Er's ancestor. However, after he was ambushed by his close friend and his own son, he was badly injured and knew he doesn't have much time left. The only for him to keep his soul alive was through his Small World.
At that time, he was already an Immortal Emperor Realm martial artist. His small world has already given birth to lives. He used a secret technique to force out the small world from inside his body, giving birth to a new world in the Lower Realm, the Holy Beast Continent.
