This discussion will be hampered without diagrams, but I’m not about to draw anything right now. Let’s all appreciate the fundamental property of the fishnet: when viewed straight-on, they are practically invisible, and when viewed from the side, they are practically opaque.
What this does is make the fishnet-encased leg look not just more slender, but more well-defined. The subtleties of the muscles are amplified. The curves and contours of the calf and thigh are enhanced, making the resulting skinniness a healthy, athletic sort of skinny. I am, as I write this, comparing fishnets with dark stockings (all in the name of science, of course), and the difference in leg enhancement is striking.