[CentOS] About Creating a DVD iso from Cd's isos

I had tried the bash script that there is on the wiki about How to
create a DVD from Cd's I got no errors during the making of the DVD but
the size of the final DVD.iso is about 2.2 Gb
when I look into the mirrors page a DVD iso is about 4 gb 
Is theiso size correct?, or I'm doing something wrong???