It increased because industrialization & mass production came to replace lots of jobs. So a farmer for example was replaced by machines and so he and his family would have to move from a rural area (farm) to an urban area (city) to get work doing hard labor in factories. The size of cities also grew because at this time the railroads made getting from places easier than ever and allowed many people to move to cities. Cities were also were many companies would set up factories and workers moved their to be closer to their jobs
