What enabled rome to become such a large city?

Rome was able to gain its empire in large part by extending some form of citizenship to many of the people it conquered. Military expansion drove economic development, bringing enslaved people and loot back to Rome, which in turn transformed the city of Rome and Roman culture.

Explanation:

Prices of products became more affordable as a result of new forms of TRANSPORTATION, which got things to market faster and cheaper
In the past, the transportation process could take up to one whole month when delivering goods. Now, as long as you're in the same country, you ould get it on 1-2 days.

Hope this helps