[CentOS] 5.5 ISO size vs RHEL

Hi

We use CentOS and RHEL, the 5.5 RHEL ISO for x86_64 is 3.7GB (**), the CentOS one 
is 4602MB (***) split over two DVDs.  Is this reasonable and correct?  Any ideas 
why would there be such a discrepancy if they are built from the same (or very 
similar) source?

Regards
Anthony Caetano

**  the md5sum checks out, and RHN lists the size as 3,532 MB

*** CentOS-5.5-x86_64-bin-DVD-1of2.iso + CentOS-5.5-x86_64-bin-DVD-2of2.iso 
(according to ftp.heanet.ie:/pub/centos/5.5/isos/x86_64 )