Originally Posted by Booga
The Aztecs Empire was based on fear, spread partially through human sacrifice, and attrition through constant low-intensity warfare. In that sort of situation you find many people willing to switch sides for the promise of protection from the authorities.
As for remarkably few men - the Spanish usually had plenty of men, but the vast majority were native auxiliaries. They often could field as many soldiers as the Aztecs could while only having several hundred actual conquistadores.