Rome is much smaller than the cities you mentioned.
the conurbation is smaller, the city of rome itself is in fact larger than paris and london, I believe.
Conversely, both Rome itself and its urban area have far fewer people than either Paris or London, which combined with the larger area, makes the density even less of a problem.
I don't know, man- Rome proper's population is larger than Paris', with approximately 2.9 million residents.