I've been doing some testing on a few strings and found that a D'Addario NW068 felt a lot lighter than a NW066. I checked the gauge with a pair of vernier callipers, no mistake, the strings were spot on 1.68mm and 1.73mm as they should be. Coiling the strings up and squeezing them the NW068 felt like more tension, again as it should be.
On the guitar however, or held in the fingers side by side, the NW066 feels thicker. On closer inspection it is clear that the winding itself is a thicker diameter on the NW066. As for playability the 68 seems a lot more comfortable and lighter but still performs better and doesn't go as sharp on the pick attack.