"So if the richer nations are more efficient, why do they also have a bigger footprint? Shouldn't China be considered more efficient since it supports a billion people with the same size footprint as the United States?"

No, China is *creating* fewer resources per person than the United States. Or, said another way, China is "less efficiently" utilizing a virtually unlimited supply of natural resources.

"Ecological footprinting measures natural resources -- air, land, water, lumber, food."

You are assuming that natural resources, like the ones listed above, are somehow "used up" by nations like the United States. I have explained how, for example, energy and usable land can be created, indeed will be created, virtually without limit. It is precisely those countries creating a bigger "ecological footprint" per person which are making these resources more and more available. Since the potential to access these "natural resources" exist without practical limit in the universe, the issue is who is able to create access to more and more of these available resources.

Those countries with higher average per capita income have higher incomes precisely because they are creating greater access to ultimately unlimited resource supplies.

What natural resource, essential for human happiness and survival (with the possible exception of ethical behavior) is not expected to be created in sufficient quantities to enable ultimately unlimited growth in human potential?

Resources are not lacking. Knowledge is. And knowledge is being created, for the most part, in advanced Western Industrialized countries.

What critical resource is really being "used up" or consumed, with no possible substitutes that will not be even better?