Okay "uncle" bottle caps it is and not only 2-liter bottle caps, less ounce/ml bottles use basically the same caps.

Now back on topic.

Looks like fine wire ground electrodes are better than fat ones for a good spark. Probably they made them bigger to get longer life out of them and most likely from consumer complaints and the competition claims of longer life as compared to another brand. Fine wire and precious metal center electrodes are still better for spark plug performance IMO.