According to this this site (warning - PDF) the Tokyo-Yokohama metropolitan area has a population of over 33,000,000. It even says that it could be as high as 41,000,000.
Does that have any significance? Certainly in Britain, the presence of abscence of a cathedral ceased to be relevant well over a century ago.
From Wikipedia:
In conurbation terms, Wikipedia lists the Tokya metropolitan area #1 with 33 million, (but could be as high as 41 million), and the Mexico City metropolitan area as # 2 with 26 million (but could be as high as 35 million).
All this stuff is why I’ve never thought city size statistics had much meaning - well, beyond just letting you say, “Wow, that Tokyo-Yokohama’s awful big!” These questions are incredibly complicated, as there is no real international standard defining how to calculate a city’s population; obviously, urban areas tend to ooze beyond official borders, and so the size of an “official” city is pretty meaningless. On the other hand, what one country declares a city or metropolitan area will not match what another country does; methods of enumerating population no doubt differ from place to place; many of the most rapidly growing cities nowadays are in developing countries and statistics about their size (which become outdated very quickly anyway) are extremely tenuous, since often little data of that sort is gathered at all, and enough of the population is in some sort of marginal living arrangement that huge portions of the population can’t be counted at all.
As the overlaps between Tokyo-Yokohama and Mexico D.F.'s population guesstimates show, even in a place like Japan a population can’t be assessed perfectly; if Mexico D.F., in reality, is on the high end and Tokyo-Yokohama’s on the low end, then the order could be reversed.
Since the OP was so obviously inadequate to define the problem and the dude hasn’t bothered to even pop back in and define it precisely, I won’t even hazard a guess. But most of the fastest growing cities in the world right now are in developing countries where farming is becoming insufficient to sustain a decent life and so people flock to places like Dar-es-Salaam in search of a better life.
No, the presence of a cathedral isn’t significant. It’s relevant only in an ecclesiastical sense to represent the “home” church of the bishop of a diocese, in which his cathedra, or throne, is placed. The greater Sydney region has Catholic cathedrals in the centre of the city, Parramatta, St Ives, Redfern and Darlington.