[tz] About the core city of a tz region

I am curious about the protocol that tz uses to specify the name of each tz region. Is the core city in a region always the most populous city in the tz region? And what is done if a smaller city's population overtakes the named city? Is the zone renamed? Also, since historical data (pre-1970) is now maintained only for the named city in a region, does it not make sense to use the population as of 1970, since the historical records actually affected more people at that time?