Compression must be distributed in a controlled manner
The purpose of a compression stocking is not simply to be tight. Medical compression works with a controlled pressure distribution.
The highest pressure is around the ankle and gradually decreases towards the knee. This is also known as graduated compression.
If a stocking does not fit snugly on the lower leg, this pressure distribution can change. As a result, the compression feels less comfortable and the support is less precise.
Every lower leg is different
Many athletes have strong calves, narrow ankles, or longer lower legs. Therefore, a general size like S, M, or L is not always enough to determine how a compression stocking actually fits.
For example, two athletes with the same shoe size can have a completely different calf circumference.
That is precisely why fit is much more important for compression stockings than for regular sports socks.