If I use winrar to compress 80gb of tv and movies.
You haven't gained anything by doing so since video is already compressed. Compressing data that is already compressed will usually make it slightly larger - or if you're lucky maybe you'll save like 1 megabyte space, not really anything worth the trouble.
Then can I compress it further by making it an iso?
ISO is not compression.
Zip bombs wouldn't apply here, from what I understand to create those you'd have to start with uncompressed data. The typical example is using a bunch of zeroes as the starting point, not only is that compressible but the compression algorithm will look for redundancies in the data to increase the compression efficiency (a bunch of zeroes would have tons of redundancy).
Stackoverflow has a good discussion on it https://stackoverflow.com/questions/1459673/how-does-one-make-a-zip-bomb