This example is Dunkin Donuts, but I've seen it in other places as well.
If you sell 2 sizes, the biggest one is "large", and the smallest one is "small".
If you sell 3 sizes, the biggest one is "large", the smallest one is "small", and the one in the middle is "medium".
It's impossible for your smallest one to be "medium".
n. pl. me·di·a (-dē-ə) or me·di·ums
1. Something, such as an intermediate course of action, that occupies a position or represents a condition midway between extremes.
If you sell 2 sizes, the biggest one is "large", and the smallest one is "small".
If you sell 3 sizes, the biggest one is "large", the smallest one is "small", and the one in the middle is "medium".
It's impossible for your smallest one to be "medium".
me·di·um
(mē′dē-əm)n. pl. me·di·a (-dē-ə) or me·di·ums
1. Something, such as an intermediate course of action, that occupies a position or represents a condition midway between extremes.