Greece's geography did not allow it to be an empire like the Persians or some of the kingdoms of the ancient Near East. Greece was divided by vast bodies of water. For example, on the main land, you had cities like Athens, Thebes, and Sparta. Across the Aegean, you had other cities that were equally impressive, such as Miletus. There were also many islands as well such as Chios, Samos, Lesbos, and many more. So, the sea itself helped the Greeks to form city states. Forming an empire would have been very difficult.
Another important feature of the geography is the mountainous terrain. In other words, each city was separated by mountains. This natural barrier also kept the Greeks apart (and also protected them from would be invaders). So, once again the geography of the land helped the Greeks create cities instead of a unified empire.