Doubts in Exp dump size

Hi All,

 DB Version is 9.2.0.8.0 .Using Traditional Export.Taken export for the below table and got the filesize of the dump file as below. its 11GB.But when i checked the dba_segments size its showing 23GB.Can you please explain why i am getting the dump size less than the table size?Is this the expected behaviour?

-rw-r--r--    1 oracle   oinstall       5973 Oct 28 13:02 archive.log
-rw-r--r--    1 oracle   oinstall 11090046976 Oct 28 13:02 archive.dmp

 select sum(BYTES)/1024/1024/1024 from dba_segments where owner= 'BILL' and SEGMENT_NAME='ARCHIVE_BALANCES';

SUM(BYTES)/1024/1024/1024
-------------------------
               23.0195313

select sum(BYTES)/1024/1024/1024 from dba_extents where owner= 'BILL' and SEGMENT_NAME='ARCHIVE_BALANCES';