If they don't buy the caffeine sensitivity, there is another issue at hand.  For espresso drinks the difference between a small and a medium often isn't in the amount of espresso. My shop gives 2 shots for all drink sizes. So the size is mainly determined by the dairy or water added. This, naturally, causes a medium to be more dilute than the small, altering the flavor. I find the mediums too dilute/sweet for my palette. I want coffee, not dessert.    Get what you asked for OP!