Southeast Asian mandalas are examples of hegemonic empires, where direct central power do not reach far away from the capital, and other cities are highly autonomous and only loosely controlled by vassal relationships. Thus something like Ayutthaya can both be arguably described as a city-state as well as an empire.
Please do read up on mandalas. It will answer all your queries. But first you need to discard your preconceived notions of what a state or empire should be.
Please do read up on mandalas. It will answer all your queries. But first you need to discard your preconceived notions of what a state or empire should be.