Their populations are still growing, though, and the Chinese and Indian populations, for example, are already at a billion each. I hope they don't get to the same point we are at, wanting two cars in every garage. That could definitely drive up the cost of gasoline.
If the population density is really high, then it makes more sense to build mass transit. Why buy a car if you're going to be stuck in the traffic at 5mph? And if the price of gas goes up, it'd generate more incentive to develop and use alternative fuel. For an example if we increase cost of electricity by 3x, you'd see solar panels going up on everyone's roofs.