How did the numbers of Spanish conquistadors compare armies of the Aztecs and Incas?

In the early 1500s, Spanish forces sailed across the Pacific and conquered the Aztec and Incan civilizations, even though the invading armies were greatly outnumbered by the indigenous population. This conquest was due, in part, to differences in technology and experience. Yet in the long term, Hernán Cortés' victory over the Aztecs and Francisco Pizarro's conquest of the Incas were the result of something that couldn't be seen by the naked eye.