Further compression of disk image files

"When you perform a Get Info on the original folder or disk, how big is it? What's the current compression ratio?

Chances are you could ZIP or RAR it, and get 3% - 5% compression... But is it really worth it? You'll spend an additional half hour, to hour attempting to compress already compressed data.

1) Try to create a sparse bundle that contains your information, and see if that is any smaller...
2) Does it have to be a disk image? If you have a target size, would ZIPing or RARing that data work better?"