that's an insanely huge city :-O
13,4 million of people (2001) lives in the metropolitan area called "Gran Buenos Aires". It means The city Buenos Aires and around metropolitan area. 1/4 of the country lives here 
1/4 is a striking proportion - just think, for instance, about political weighings...

Metro São Paulo holds "only" ~1/9 of Brazil's population. And BTW, I do get bewildered at times with its dimensions

And it's nice to look at how La Plata (Chulk's "small" city) has very nearly fused into Buenos Aires conurbation, but not quite...