Why was rome able to have larger cities and a larger population than greece?

First, they imported a LOT of food from other parts of the empire. Rome is now built on the remains of Roman food packaging material: broken up pottery. Romans mainly lived on what is know as the mediterranean triangle: Wheat, wine and olive oil. Bread made up 70 to 80% of most of the Romans diet. Three kinds of food that are pretty easy to transport over longer distances. There were huge imports of grain from north Africa and Egypt. Rome lived on that grain. Wine and olive oil came from almost everywhere.
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