Arithmetic, Overpopulation, and J. G. Ballard's “Billennium”

Let's see … In the overpopulation tale “Billennium” by J. G. Ballard, the world population is a mere twenty billion. In order to be sure, I checked the J. G. Ballard Short Fiction Concordance and found the following quote from “Billennium” (under “four”):

Rossiter paused, lowering his voice. ‘Four per cent. Eight hundred million extra …

If four per cent of x is eight hundred million, x must be twenty billion. I also noticed the following, also from “Billennium”:

It couldn’t be done they all said, no one could stand living in only four square metres, it was enough doors would open outwards. Four square metres was here to stay.

At four square metres per person, twenty billion persons could be put into an area of eighty thousand square kilometers — about the size of Maine.

I'd like to know who (or what) has taken over the rest of Earth.

That's an idea for a story. At first it looks like a standard overpopulated dystopia and then the protagonist realizes that the numbers don't add up …