There are three factors at play: population size, population lifestyle, and efficiency of resource use. It is customary to completely ignore population size as

There are three factors at play: population size, population lifestyle, and efficiency of resource use. It is customary to completely ignore population size as a factor, as long as lifestyles or resource use could be more eco-friendly (commute with bicycles, rebuild infrastructure to make that possible, re-zone cities, change diets, make industries and farming greener, etc..) This is the case even though those large, society-wide transformations would take decades, and even while the population is actively being increased by immigration (most of the developed world has had below-replacement fertility for decades, so birthrates are not a factor).