If they are more populated than the chances are that they are emitting more toxins in to the atmosphere and therefore should be more active in combating climate change. The more developed the state is the more concerned that state should be about its emissions and the environment as a whole.
Developed countries are more obligated to combat climate change for several reasons. First of all, they consume more product and pollute more. The United States, Australia, and other developed countries generate more carbon emissions per capita than any developing country. If people in developed countries lived in the way that average people from developing countries live, we would not have the carbon emissons problem to begin with.