In the last chapter of My Side of the Mountain, Sam's solitary life in the wilderness changes with the arrival of his family, who have come to the Catskill Mountains to make a new life for themselves together with Sam on Grandfather Gribley's farm. Upon seeing them, Sam exclaims proudly, "Wow, all of New York!"
Sam's family was certainly a lot smaller than the population of New York City, but after living for a year with very little human interaction, Sam evidently found all the activity and noise caused by the new arrivals reminiscent of life in the city.
Later in the chapter, Sam refers to his family as a "city of people:"
The next day I took John, Jim and Hank out into the mountain meadows with Frightful to see if we could not round up enough food to feed this city of people. We did pretty well.
In creating a title for this chapter, the author skillfully uses a figure of speech known as synecdoche wherein the whole (city) is used to represent a part (Sam's family, who were among the population of that city). If we restate her title without the synecdoche, as the example below indicates, its literal meaning becomes apparent.
Example: My family, from the city, comes to live with me.