If you add up the numbers of the countries you mentioned (together with 1% of India or China), you'll realize that it's smaller than the US population...
US: 331.9 million
UK: 67.33 million
AU: 25.69 million
CA: 38.25 million
NZ: 5.123 million
IN: 1.408 billion * 0.01 = 14m
Point is some very small percentage of the rest of the world is going to dwarf the US. Yes the US will be the single biggest demographic but less than 50% of the total.
I haven't been able to find many solid stats, but eg English language Wikipedia, the US has 22k active editors out of 59k total.
US: 331.9 million
UK: 67.33 million AU: 25.69 million CA: 38.25 million NZ: 5.123 million IN: 1.408 billion * 0.01 = 14m