Absolute GHG emissions and impact are higher for higher incomes. GHG emissions are a function of wealth not geography. Urban areas have much higher concentraions of wealth. They also outsource much of their GHG creation to rural areas but are still the cause/consumer of it (concrete, energy, minerals, construction materials).
Also the perceived reduction in GHG emissions for urban areas is only true of late stage developed nations. Developing nations, those building out new cities, do not share the per capita efficiencies of full developed nations. The reason for this is that perceived per capita GHG reductions in developed nations were paid for by high GHG emissions of their past. New data on urban sprawl (sub-urbanization) sheds doubt on the idea that even developed nations cities per capita GHG emissions are lower than rural areas.
Ultimately, urbanization is a bulk consumer of Earth's natural resources and not a producer of those inputs.
Put simply: Cities are eating the Earth.
EDIT: Added point about sub-urban development negating efficiencies.