How do you know that?
Comfortable for an hour or two. Small/ close fit. Once positioned you won't be forever adjusting yourself or grabbing your crotch like a smooth criminal. You can run really well with this. I think anyone other than a very slim man or youth should go for large or larger.
Quite fancy trying one, - anyone got one for sale they don't need anymore?
seriously, you want to try a used box !!!