Bought the small, measured my thigh as a bit bigger than 16", top doesn't even begin to grab my thigh, bottom velcro elastic is too long to really grab my calf, so THEY SLIDE DOWN. Don't play snake in these.
Otherwise, great knee pads. Not quite as large as the Eclipse ones, but still lots of padding, and I love the sleeve under the straps, it's a nice fabric and doesn't irritate the back of your knee. However, if you look at the picture of the rear of the pads, you'll see that there is a black elastic patch sewn into the middle of the top strap, there are two FAT stitching seams on the inside of that, which could get irritating if these actually grabbed your thigh, I can't comment, as they are too big for me.
They didn't slide very well directly on grass, if you're the kind of lunatic who does that to your pads.