[tpeo]

I am not a historian, but the Romans seem to have grown at the expense of weaker and more disorganized foes, who either didn't have any form of centralized leadership prior to contact with the Romans (like the Gauls and Iberians) or who were pitched against eachother in civil war (like the Egyptians). When fighting against similarly sized centralized states, like Parthia (or Carthage, in the early republic), they had a much harder time winning anything out of them. I think they mainly exploited a preexisting assymetry.