I'd be interested in more info on these things. I read at their website about how they should fit (basically UNsnug). Today I tried some on (I usually take a 7.5 size). Well, the mediums list a size comparison of 6-7 and the large lists 8-9.

In other words, they don't seem to cover 7.5 and, sure enough, when I tried on the medium size, they seemed to cause pressure on my toes when moving while the large size seemed to leave me swimming in extra space!