Compressing DMGs?

I've noticed a lot lately that I'll download a 20MB DMG with a 50MB app in it, I was wonder how the devs do this, as if I could do something like this with the files on my Mac I could really save some space.

Thanks in Advance, any help would be appreciated

this may help, but try it out

http://www.macupdate.com/info.php/id/15740/freedmg#descContainer_link

Thanks for the help, but the download link isn't working

found the developer

http://www.kelleycomputing.net:16080/freedmg/

Ah thank you again louishen