Re: Welcome to MegaCity
The human population right now is increasing, but no longer exponentially. Therefore, growth has slowed. Which is different from saying that growth has decreased. 
What you will see, if you do see anything, is the transition of smaller towns into cities, and then into big cities.
Overall, though, it seems that the human race is subject to the forces of a carrying capacity. Population ecologists are not sure, but they think we might be reaching the carrying capacity.
The theory of a megacity is absolutely improbable. You would need massive division of labor to manage the different areas of the city. And I doubt the infrastructure is there to develop one present city into such a big city.
I think your UN people need to talk to some population biologists. Oh right, the government people never listen to ecologists. They think they’re smarter.