You could enable file system compression if that's an NTFS drive and see how much space you can reclaim this way if you want to keep stuff directly usable without zips. If most of the stuff on it is rarely used and highly compressible, then zips may be a better idea.
It depends what you store on this volume. Videos, music, photos are not compressable - they are already compressed.
Another question you have to answer is how ofthen you access these files, and what apps you use for them.