Mumbai was the Indian city proper with the largest population in the 2011 census.
The 2011 Census of India recorded Mumbai’s population at 12,442,373 within the city’s municipal boundaries. That placed it ahead of Delhi, whose city population was 11,034,555, as well as Bengaluru and Kolkata. These figures compare city proper or municipal populations, not the much broader metropolitan regions surrounding each city.
Mumbai’s unusually large population reflects its role as Maharashtra’s capital, India’s financial centre, and a major port on the Arabian Sea. Its urban area includes densely settled districts on Mumbai Island and the northern suburbs, while the wider Mumbai Metropolitan Region extends well beyond the city itself.
A common mix-up comes from comparing urban agglomerations instead of city boundaries. Greater Mumbai’s 2011 urban-agglomeration population was about 18.4 million, while Delhi’s was about 16.3 million. Either way, under the census city-proper comparison used in this question, Mumbai ranks first.