It's a bit of a weird semi-pedantic stats claim, they're basically counting "largest lowest level of government". Birmingham is a large unitary authority, there

It's a bit of a weird semi-pedantic stats claim, they're basically counting "largest lowest level of government". Birmingham is a large unitary authority, there are no district or borough councils below it. Looking at Wikipedia Istanbul seems to be split into districts which have their own local governments (similar to London with its boroughs). So they're counting the 514,900 population of Başakşehir or 259,146 of Hackney vs the 1.1 million of Birmingham, rather than the total populations of Istanbul or Greater London.