Do you wish to show the one-from-many nature of the conurbation at the main map's scale, as well as in the inset? If so, your symbol could be faceted instead of a simple or concentric circle/square shape. In the inset, is it important to show the original multiple city centers within the overall megalopolis? In midlevel detail of old cities, the original center often shows as a more complex knot of denser streets, perhaps at a different orientation than the more recent parts, which might be a net of wider connections. One doesn't have to draw every street, to get a sense of the structure of the network.

If you take that path, the original distinct city centers would show as knots of denser streets, in a complex spider's web of the "modern" city. Some of the overall orientation of streets could be synchronized with the original highways connecting the distinct cities. If your history includes ambitious builders, or catastrophic disasters, you could show "urban renewal" by roughing in this multi-centered spiders' web, then clearing one or more bits, putting in latter-day palaces, parks, parade grounds, and whatnot.

Another way to show growth of an old city is to put in bits or great swaths of some early walls, swallowed by growth. Labels like The Old Fort Quarter can make it more obvious what you're showing. The wal makes a god break point from tangled old-old streets to whatever kind of "early suburban" growth you imagine took place. What kind of scale are you contemplating for the inset?