[hevi_jos]

You are mixing things that don't have anything to do with each other. Two empires separated by more than a thousand of years.

While talking about an Empire expansion you omit the biggest factor: You expand or grow conquering-stealing-destroying others and the limit for this is geographic, not taxes.

In the particular case of the Roman Empire they needed to communicate all the provinces, and they did that by sea, mainly using the Mediterranean Sea. There was no train, like there is today(which is a tremendous difference as it is several orders of magnitude of difference transporting things).

Once you conquered all the Mediterranean Sea, there was nothing else to conquer, communicating by land was super expensive. Today it is the size of the world, until we get out to other planets.

Also in today's world there are weapons of mass destruction like nuclear weapons. If you try to grow at the expense of others you could get destroyed.