8mm spark plug wires are a heavier gage wire than the 7mm type. However, there is more to it than just the difference in the thickness. The thicker wire has a thicker core. The core is the part of the wire that brings power to the spark plugs. A more robust core provides a lower resistance path for the power, thereby allowing more power to reach the spark plugs. Additionally, the 8mm wires have a thicker outer coating that keeps the core safe from damage. These wires also have a thicker layer of suppression material.
For more info, see How to Inspect Your Spark Plug Wires (and Why It’s Important)