Could a decrease in population be a factor leading to China having a lower environmental impact?
Less people and lower GDP would probably mean lower energy consumption (or at least slower growth), which China produces in large part with coal. Less overall consumption in food and other products could lessen environmental impact in other ways.
This article focus on GDP. The earth isn't shrinking because human population shrink. If the pie is earth's carrying capacity, it means a bit more pie for each inhabitant.