If you want to compress video files, you’ll need to reencode them. Maybe using something like HEVC (High Efficiency Video Codec). But for 80GB of videos, you’ll be there for a while and probably won’t shrink them enough to be worth it. It would likely take less time to simply re-download the files later, even with a mediocre internet connection. In practical terms, you won’t get that 80GB to be any smaller.

ISOs don’t compress anything, as far as I know, or at least not by default. I think they’re basically just a container.

To reencode your videos, you can use the free HandBrake.