With higher birth rates among Hispanic and Asian New Yorkers, immigrants continuing to gravitate to New York City and a housing boom transforming all five boroughs, the city is struggling to cope with a phenomenon that few other cities in the Northeast or Midwest now face: a growing population. It is expected to pass nine million by 2020. New York might need an extra million or so slices of cake for its 400th birthday party in 2025. Estimated today at a record 8.2 million, the population is expected to reach nearly 9.4 million in 2025. But that projected growth poses...