This definition?
. the administrative area of London, consisting of the City of London and 32 boroughs (13 Inner London boroughs and 19 Outer London boroughs): formed in 1965 from the City, parts of Surrey, Kent, Essex, and Hertfordshire, and almost all of Middlesex, and abolished for administrative purposes in 1996: a Mayor of London and a new London Assembly took office in 2000. Pop: 7 387 900 (2003 est). Area: 1579 sq km (610 sq miles)
That's bigger than a lot of sovereign states. How can you compare it with the City of Southampton?