The Chinese population had crashed at the end of the Song dynasty in 1279, from a high of 200 million to 120 million by the beginning of the Ming. It was the greatest population collapse in human history in absolute numbers. The Ming only recovered by 1600.
During its age of discovery in the early 1400s, China had a lower population density than England when it started settler colonies in the New World. Hence the trope that China had plenty of people to send abroad is not backed by evidence. It would be like Europe colonizing the world after the Black Death. There were plenty of abandoned farmland in need of reclaimation at home.
The Spanish conquest was not settler colonization. The Conquistadores didn’t go into the American wilderness to farm, they conquered densely populated agrarian empires to exploit their labor. It was similar to the First Crusade were the Crusaders went and established kingdoms in the holy land, not a colony for growing settler population. The whole convert the heathens angle was part of the scheme. For the conquered to accept Christ also means they accept the overlordship of Christ’s annointed King of Spain.
For Ming China to do what the Spanish did would require them to sail around Africa, cross the mid-Atlantic and trace the coast of Brazil to Panama, from there armies would invade Peru and Mexico and install puppet rulers. Crossing the Pacific and back would have to wait until they figure out the currents.