The sizes run differently for each company, it's always better to have shorts that are a little big than too small. And i think they are better, if you are constricted in any way while you grapple than you are being hindered. With grappling shorts they all almost have a crotch that is flexible and durable as well, along with a split seem from the out side of both legs affering full range of motion.
Buy some they are worth it, also wear a cup and you won't smash your nuts while throwing an armbar.
only 2 men have sacrficed their lives for you, Jesus Christ and the American soldier