How was Peter the Great great?

Peter the Great was great in the sense that he modernized Russia according to Western custom and expanded its borders. Peter the Great acquired Kiev from Poland and he gained ports on the Black Sea from the Ottoman Empire. He gained new territory around the Caspian Sea as well. Peter the Great built the Russian navy according to Western European traditions and ended arranged marriages in the Russian nobility. He also established the Julian calendar in Russia in 1700. Peter the Great established St. Petersburg as the new capital of Russia and even hired designers from Western Europe to design many of the official buildings. Peter the Great also successfully went to war against Sweden and took what would later be known as the Baltic States.  
Domestically, Peter the Great tried to create a Russian nobility that valued merit and service more than hereditary position. His Table of Ranks lasted until the fall of the Romanov family in 1917. He also changed taxation policy in order to gain tax money from all classes of Russian society. These domestic changes are secondary to Peter's territorial acquisitions as to why Peter the Great is "great." He is best remembered for modernizing the Russian state and making it a player in Western European politics.