The .MYI is a MYIndex file for the MyISAM table type (the default MySQL table) -- fyi, the .MYD is the data .. So I guess to make the .MYI file smaller you would want to reduce the number of indexes in your table.. There are a few articles on this site regarding indexes, proper usage, etc that you may want to look at.
John
[ black dog, brown dog - you decide @ smashedbug.com ]