Wikipedia says the following:
During leap years on the Hebrew calendar, Purim is celebrated in the second month of Adar... The 14th of the first Adar is then called Purim Katan ("Little Purim" in Hebrew) and the 15th is Shushan Purim Katan, for which there are no set observances but it has a minor holiday aspect to it. The distinctions between the first and the second Purim in leap years are mentioned in the Mishnah. Certain prayers like Tachanun, Keil Erech Apayim (when 15 Adar I is a Monday or Thursday) and Lam'nazteach (Psalm 20) are omitted during the service. When 15th Adar I is on Shabbat, "Av Harachamim" is omitted. When either 13th or 15th Adar I falls on Shabbat, "Tzidkas'cha" is omitted at Mincha. Fasting is prohibited.
Why is Purim that falls in Adar I called Purim Katan; yet, in Adar II we don't call it Purim Gadol?