Depends what is in the ISO. An ISO is just a CD or DVD image. If the DVD contained compressible data, yes, eg lots of old-format BMP files, maybe you could compress it that much. But real-world ISOs tend to be full of incompressible data such as program code and/or multimedia content. Data compression programs are not magic wands, and they work best on data which contains a lot of redundant information. Game or DVD ISOs do not contain much of that. Think about it. If Winrar was a magic file shrinker, all content would be compressed down to 1 bit, and you wouldn't need DVDs at all.
