to be more precise, you've to use the myisampack utility
alternatively, you've the new storage engine called "archive" which renders roughly the same service and moreover prtects your data from manipulations.
some sample data :
The Archive table is almost 75% smaller in size then the MyISAM table and almost 83% smaller in size then the transactional InnoDB engine
Even with compression applied, the Archive table is still 7% smaller than the packed MyISAM table
see
http://dev.mysql.com/tech-resources/articles/storage-engine.html