If we take Sydney as an example, what sound economic reasons give it a population of 0 million rather than 0 million? Urbanisation is just the consequences of the population clustering, but when we have so few places to cluster around as a result of the way we've not only organised our government functions (the public service is the biggest employer in the country by far), but the way we've encouraged capital cities to monopolise metropolitan type industry - our lack of large cities seems to be more driven by historical inertia than any type of necessarily rationale behaviour.