MySQL: reducing ibdata file size for MyISAM tables
MyISAM tables do not use ibdata files, so they won't affect the size of ibdata files.

From the MySQL Docs, here is how MyISAM stores data on disk:


  Each MyISAM table is stored on disk in three files. The files have names that begin with the table name and have an extension to indicate the file type. An .frm file stores the table format. The data file has an .MYD (MYData) extension. The index file has an .MYI (MYIndex) extension.