I've merged your two threads, as there is no need for two tippinndown...same subject here.
To reduce the size of the ISO, open it with DVDShrink. DVDShrink is a free compression program for dvd video, and it only works on dvd video, so I hope that is what you have.
Hit Ctrl I once you have Shrink running. That will allow you to navigate to the file. Change the output settings in Shrink to an ISO if that is what you want to have in the end.