Can it be a simply stupid advertising ploy? Surely asking for an extra large unhealthy meal will stimulate those guilt centres in the brain more than asking for a large unhealthy meal, regardless of the size of the meal itself. I'm inclined to agree that "Medium" is the wrong term. Maybe "Regular" is more apt, on which note ...

the smallest condom size you will find is "Regular". After all, who would want to buy a "Small"?