Leg measurements are as follows:
Upper Thigh Circumference: 26"
Middle Thigh Circumference: 24"
Lower Thigh Circumference: 21"
Calf Circumference: 18.5"
From the images below you can see that my thighs are long and is only a difference from from lower thigh to upper thing of 5 inches.
I also has very long legs at 37" inseam so we also want to show that they will be long enough even when you want to wear your short skirts or dresses.
As you can see with the Glamory Comfort 20 size Medium 40-42, they do not fit. They are not suppose though either.
The best way to ensure the proper sizing is to actually measure your inseam and the thickest part of your thigh. Above is the sizing chart based on those measurements. Because of the isometrics of hosiery you want to use the greater measurement of the two for the correct size.
We are always hearing how women don't like to wear thigh highs because they just roll down. Well maybe you are just wearing the wrong size.