East India Company had replace Surat port with Bombay and in the nineteenth century it becomes an important western administrative port and industrial centre. Because of this more opportunities for trade and commerce, and employment was increased gradually. So, Bombay became an attractive destination for migrants seeking jobs.

So the major reasons for expansion of Bombay's population were:

1. Trade: Bombay became the capital of the Bombay Presidency in 1819, after the defeat in the Anglo-Maratha War. The city quickly expanded, with the growth of trade in cotton and opium, large communities of traders and bankers as artisans and shopkeepers came to settle in Bombay.

2. The establishment of textile mills: Establishment of textile mills led to a fresh surge in migration. The first cotton textile Bombay was established in 1854. By 1921, there were 85 cotton employing about 146,000 workers.

3. Railways: Bombay dominated the maritime trade of India till well into the 20th century, was also head of two major railways. The railways encouraged an even higher scale of migration into the city.