huge sql dump even after deleting large number of rows of data

The size of sql dump is same(30GB) even if I delete large number of rows from mysql (myisam) table

note: this variabe is innodb_file_per_table ON

mysql> delete from radacct where YEAR(acctstarttime)='2014';
Query OK, 1963534 rows affected (1 hour 30.58 sec)