The article Rise of the Inca says the growth of the Inca Empire was meteoric.  By 1532, it stretched from what is now southern Colombia to central Chile. With a popuation of 10-12 million, it was the largest empire in the Americas.

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There were several things the Incas did that helped secure them a domain as vast as the Roman Empire. They worked very hard in diplomacy, negotiating with people and to work out amicable relationships through gift exchanges, marital exchanges, or political alliances. Failing that, they would threaten and only later undertake military conquest.

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The Inca gave very valuable textiles and other gifts to those they conquered and threw feasts for them. They presented themselves as benefactors to the population by giving gifts on a grand scale. One of the names the Inca ruler took translates from the Quechua as "lover and benefactor of the poor." This was manifested in practical terms by giving the people all the food and drink they needed while they were carrying out their labors on behalf of the state.

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Another innovative thing the Incas did that built on existing systems was to create a vast network of roads, some 40,000 kilometers which joined 2,000 provincial way stations or administrative or production centers.