Final answer:
During the Gilded Age, the size of cities increased due to rapid industrialization, new technologies that enabled urban factory construction, and the migration of workers and immigrants seeking employment in the burgeoning industrial sector.
Step-by-step explanation:
The size of cities increased during the Gilded Age primarily as a result of rapid industrialization and urbanization. This era saw a significant shift from agricultural economies to industrial ones, spurred on by technological advancements like the steam engine, which allowed factories to be built away from rivers and closer to urban centers. The creation of factories in cities offered a myriad of employment opportunities, drawing both rural populations seeking work and immigrants who arrived due to a variety of push factors, such as famine or persecution in their home countries.
As factories began to power their operations with steam and then electricity, they were no longer bound by the limitations of waterpower, thus permitting them to operate continuously and employ vast numbers of workers. Cities like New York and Chicago became filled with smokestacks, skyscrapers, and elevated railroads, signifying their industrial might. Despite the challenging working conditions, including long hours and dangerous environments, people were attracted to the cities for the promise of steady wages as opposed to the uncertain prospects of agriculture.