Sumer Was About the Size of Belgium

This first civilization came to be known as Sumer. By about the year 3000 [B.C.E.], a city called Uruk near the mouth of the Euphrates River, just inland of the head of the Persian Gulf, had eighty thousand residents. A thousand years later Iraq, the land along the Euphrates and its sister stream, the Tigris, would be named for this early metropolis of Uruk. Sharing the land of Sumer, about the size of [modern] Belgium, with a dozen other city-states, Uruk was not always the foremost among its rivals in the land. But for most of its existence, spanning the two millennia of the Sumerian world, Uruk was the greatest city on earth.

— Land Between The Rivers, Prologue

A dozen (small) cities in such a small area. Every square meter of arable land was claimed and constantly fought over. I do not see much of this in fantasy roleplaying campaigns.