China and India have about 1.4 billion people each, not even counting their diaspora. The next closest in population is the USA at 320 million. If you're doing rep by pop, they win.
I don't see a world government being the old nonsense of rp by pop. We would need to advance to a system where we could have what we might call rep by need and strive to level the playing field a little better than it is just now.