Does a numeric field always use the same disk space? When you have a numeric field in MySQL(MYISAM) (E.g. Integer) will it always use the same disk space, no matter what number is actually stored?
MySQL's integers take 4 bytes, regardless of the value you store.  (Data storage requirements)

MyISAM supports compression, but only with a compressed row format, which makes the table read-only. I don't think I've ever worked on an application where a read-only table was appropriate.