According to official population statistics, in 2021 the number of people per square kilometre (per square mile) in each of the four cities was as follows:

Birmingham: 4261 per sq km (11036 per sq mi)

Manchester: 4766 per sq km (12344 per sq mi)

Southampton: 5050 per sq km (13079 per sq mi)

Portsmouth: 5373 per sq km (13916 per sq mi)

It is not hard to see why Portsmouth is so densely populated. Like Southampton, it is on the South Coast, a fairly prosperous part of England. Most of the city lies cramped on a small island (Portsea Island). It is extremely active economically.

Portsmouth is one of the world's best known international ports, handling both cargo and passenger ferries. It contains the largest dockyard for the Royal Navy and substantial naval construction work is undertaken here. It is home to two-thirds of the Royal Navy's surface fleet.

More Info: en.wikipedia.org