London and City of London are two different entities entirely so that comparison does not work. London at its largest refers to the 32 boroughs and City of London that makes up Greater London, not the City, to those of us who live here. At it's smallest, it still refers to most of that, not just City.
If we want to shorten the name of City of London, the short form is the City, not London. The only people who ever calls City by the name of London is people who have just learnt of the oddity.
Greater London makes up the formal boundaries of London. It's the legal and administrative boundary, and the city limits of London. Unusually, unlike the city limits of many other cities, you'll find plenty of people including people who live here who not quite consider the outer parts to be part of London. People where I live sometimes still consider it part of Surrey even though it's been part of London since 1965.
There are forests and agricultural areas almost entirely separating parts of the borough I live in from the rest of London.
Yet in other areas, the Greater London urban area expands well past what anyone would call London, including e.g. entirely separate towns in other counties, like Watford in Hertfordshire, Gravesend in Kent, Epsom and Guildford in Surrey.
So yes, these comparisons are tricky but that is the point.
E.g culturally, London is likely reasonably described as smaller than it's city limits, while indeed some other cities are often larger than them, but rarely as large as the urban area they are within. It's extremely common for towns on the outskirts of large cities to have their own separate identities and not be considered part of the adjoining city, but still form part of the same urban area.
Depending on what you want to compare and which cities you are comparing, different measures will be more or less appropriate, and may require different considerations.