Sorry, just seeing this now. Where do you get the idea that there was very little trade in the Inca Empire? Trade, including local commerce, medium-range trade, and very long-distance trade, was very common in the Inca Empire and among the conquered places. Trade has long been very important to Andean societies, and if anything being brought into the Inca Empire allowed entry into a huge and wealthy trade network. We generally say that there were no marketplaces in the Andes (Inca or otherwise) and trade was controlled by the state. That is likely a huge simplification but we don't see true markets like you find in most other cities. But that doesn't mean that there was no trade, just that there were no private/independent merchants, at least not in the sense that we normally think of when we think merchants (again, possibly a huge over-simplification).

But the cities also were administrative centres, homes of the elite and wealthy, homes of artisans, homes of priests and temples, etc. Some of the largest cities likely were quite a good size (I'm not sure about Cusco but if I believe that estimates put it at over 100,000. Chan Chan, capital of the Chimú Kingdom and eventually conquered by the Inka Empire, was at least 100,000).

Sources:

Earle, T. K. (2002). Exchange and Social Stratification in the Andes The Xauxa Case. In Empire and Domestic Economy (pp. 297-314). Springer US.

Moseley, M. E. (1997). The Incas and their ancestors: the archaeology of Peru. Thames and Hudson.