You're talking about the pomerium, the sacred boundary around the city. A promagistrate's imperium ended when he crossed the pomerium and sitting magistrates were not supposed to bring their armies across it--likewise comitia, originally an extension of military mustering, could only be held outside the pomerium. The pomerium was extremely ancient and predated both the oldest city walls and the the actual extension of the city into a sprawling urban metropolis. It was rarely extended, as extension of the pomerium ritually was the prerogative of the king. The earliest form of the pomerium probably only incorporated the Palatine, although extensions attributed to Servius Tullius extended it to several of the other hills. Sulla expanded it, as did several of the emperors, but it never attempted to even try to approach the size of the actual urban area, nor was that its purpose. The actual urban area of the city extended for miles, and the suburban regions included most of northern Latium