In terms of absolute emissions, China is indeed the biggest polluter. But that is, frankly, a naive way of measuring things. On a per-capita basis, China is actually one of the smallest polluters. On a per-capita adjusted basis, the US (and Saudi Arabia) are the world's biggest polluters. For surely every person has an equal responsibility when it comes to pollution, no?
And China's supposed pollution isn't even taking into account that most of its pollution occurs in the production of goods the rest of the world wants. I.e. if a Chinese factory didn't do the pollution, the pollution would simply occur somewhere else. Western countries (and citizens) are good at pointing the finger when, indirectly, they're the root cause of much of the pollution that occurs in these developing countries.