To create and maintain smaller cities you would need to enforce controls on population growth and distribution. Controls which are abhorred by the civilized world.
Yet, there are many instances where population growth or decline has become an issue due to economic drivers, alone. Over the last 23 years Detroit has lost 34% of its population while Austin has gained 44%. You can't just assume people will distribute evenly into nice small groupings when they are granted freedom of movement and the right to pursue happiness.
Like everything else, you build for scalability and do your best to maintain.