On earth advancements in technology, education, and especially woman's rights quickly lead to reduced or even negative population growth. So maybe interstellar civilisation won't need that many space cities
Who knows! Perhaps one of the necessary steps to conquering the distance between the stars is to develop robust robotics and the ability to somehow convert their consciousness into computer forms and cast off all the needs of their former fleshy bodies… vastly transforming the needs of such a society to primarily needing computers and energy for “sustenance”.