What led to the major expansion of Bombay's population in the mid-nineteenth century?

Bombay became the capital city of the Bombay Presidency in 1819 after the defeat of the Marathas by the British.
As the trade in opium and other trade grew, large communities of traders and bankers as well as artisans and shopkeepers came to settle in Bombay. Another important reason for the expansion of Bombay was the establishment of textile mills which also led to a fresh surge in migration.