In Megillat Esther, and elsewhere in the Torah, you find several places where a letter is written slightly larger or slightly smaller than the other letters. This is an ancient tradition, and the reason for each instance isn't always explained.
The particular ones you mentioned (Esther 9:7,9) aren't explained in any classical sources. Recently, however, it has been discovered that these letters, which occur in the section describing the hanging deaths of Haman's ten sons, may contain an uncanny hint to the Nuremberg trials in which ten Nazis were tried and hung for their anti-Semitic crimes, as follows:
As you may know, the Jewish calendar year is represented by Hebrew letters. The small letters in the names of Haman's ten sons are: "tav" "shin" "zain." The large letter is "vav." These letters represent the year 707 ("tav shin zain" equal 707) of the sixth millennium (represented by the large "vav" which equals 6). Thus you have the Jewish date 5707, or 1946 by the civil calendar. On the first of October, 1946 - 6 Tishrei 5707 on the Jewish calendar - the Nuremberg Military Tribunal tried ten Nazis and sentenced them to death by hanging for their modern "Hamanism." One of them, the notorious Julius Streiker, even cried "Purim-Fest 1946" as his cryptic last words.
- The Jewish Observer," March 1986, pp. 56-57