I think of the population/immigration growth lobby in terms of the cancer metaphor, an entity which is never satisfied, and knows no limits, spreading and ultimately destroying the body social, and itself. Thus, over in your place, Australia, Big Business says that they want a population as big as America, but if they had it, they would not stop there and would then attempt to create a population as big as China, and then the world. Still, markets would not be big enough. Only the entire multiverse, full of a transfinite number of consuming orifices would come close to giving the infinitely hungry monster what it wants.
“The United States is not “full.” In fact, it is empty. Right now, the country has about 93 people per square mile. Many, many countries are far denser than this, and not just city-states like Singapore (more than 20,000 per square mile) or small island nations like Malta (3,913 per square mile). South Korea has 1,337 people per square mile, and Belgium has 976. If you tripled the population of the United States, adding the new Americans only to the Lower 48 and leaving Alaska and Hawaii intact and unchanged, the main part of America would be only about as dense as France and less than half as dense as Germany. A transformation on that scale is almost impossible to imagine, in large part because the American political system has fallen into a state of torpor and dysfunction driven by, among other things, the absence of the shared sense of purpose that once bound the national experiment. But while contemporary politics is terrifying in certain ways, it has also opened up again the possibility of goals, and projects, and ideas — probably the biggest opportunity in a generation for new ideas to take hold. So here is one big one: a billion Americans. When America faced down Nazi Germany and the Soviet Union, we were the big dog. We had more people, more wealth, and more industrial capacity. (Back in 1938, the gross domestic product of the U.S. alone was larger than that of Germany, Japan, and Italy combined.) But against China, we are the little dog: There are more than 1 billion of them to about 330 million of us. Chinese people don’t need to become as rich as Americans for China’s overall economy to outweigh ours. If they managed to become about half as rich as we are on a per person basis, like the Bahamas or Spain, then their economy would be far larger than ours in the aggregate. To become one-third as rich as we are, like Portugal or Greece, would be enough to pull even. To stay on top, we probably need to grow the country threefold — to one billion Americans. If you tripled the population of the United States, it would still be only about as dense as France.