What are some differences in organization between the Persian Empire and the Roman Empire in terms of size?

The Persian Empire (550–330 BCE) was also called the Achaemenid Empire and was founded by Cyrus the Great. The Persian Empire spanned over 5.5 million square kilometers, or 2.1 million square miles. At the time, it was the largest empire in history and ruled through an organized centralized bureaucracy in which the king administered power through satraps, or provincial rulers. The empire had an organized civil service system and developed infrastructure such as roads and a postal system. From its founding in what is now Iran, the empire stretched through Eastern Europe, Asia Minor, Central Asia, the Caucasus, and Egypt. Its population at around 500 BCE was estimated at anywhere from 17 million to 35 million people. 
The Roman Empire (27 BCE–410 CE) controlled 5 million square kilometers or 1.93 million square miles of land at the height of its expansion (around 117 CE). Therefore, its land mass was smaller than that of the Persian Empire. However, the Roman Empire was more densely populated and contained about 56.8 million people. (The empire may have contained as many as 90 million people, which represented about 20% of the world's population at the time.) After Rome conquered territories, the territories were administered through cooperation with local officials. Local territories were allowed to maintain their own laws, and the extent of the Roman government was limited in some places. The empire's territory was vast, extending north to Britain and including western Europe, parts of eastern Europe, parts of the Middle East, modern-day Turkey, and parts of North Africa.