DMG or zip size??

I downloaded firefox there the other day and it was something like 7mb. When I mounted the dmg the actual app was 45mb... If I ever create a dmg of different files it never compresses to that extent.. (over 6 times smaller..) I have tried with pictures, movies iphoto libraries and loads of others. They all go from huge to just under huge, maybe 10 mb difference. I know that there are movie compressors out there and I use one or two, but these permanently compress them.

Is there any way of compressing files like firefox was but being able to, for example, mount a disk image and there all there in their entirety???

Regards