I don't think it was a metaphor, I grew up with it so I always imagined a bag being zipped up. That deemphasises the compression aspect though. I wonder if th

I don't think it was a metaphor, I grew up with it so I always imagined a bag being zipped up. That deemphasises the compression aspect though. I wonder if this influenced 'tarball', or maybe it's just human nature to think of objects in those terms.