The New York metropolitan area grew rapidly for several reasons: The city was the financial capital of the United States of America, which became the world's largest economy; it was the primary destination for immigrants from Europe to America; and it was a major destination for internal migration, particularly for African-Americans leaving the apartheid southern states in the first third of the 20th century.
The Toronto metropolitan area grew rapidly in the second half of the 20th century, partly after racist national immigration policies were eased, and also as jobs moved to it from Montréal due to political uncertainties in the province of Québec.
The Boston metropolitan area grew in the second half of the 20th century, as suburbanization spilled beyond the state's borders.