Over the years, as the United States grew, both economically and in the form of population, there have been increasing bureaucracy in the system.
As the population of a county grows, so does the amount of public officials who work for the country and maintain it.
Counties come under the State government and perform various tasks with elected officials including:
1. Enforcement of Law2. Conducting Elections3. Collecting local Taxes4. Spending on Public Goods etc.
Hence, as the population of the county grows so does it's budget and its overall size of administration.