He's talking about the categorization that Cosmic Ears uses.
Basically, the full and medium are the same size, except that the medium takes away the outcropping that goes underneath the legs of the antihelix. The result is a less secure fit, but it's easier to put in and take out. For the micro fit, as you can see, it doesn't even fill the entire concha; it barely covers the entrance of the external acoustic meatus and the tragus/antitragus. That makes the shell more difficult to handle, as typically, we hold the shell by the edge lining the antitragus (the antitragus creates a natural indentation in the shell) and the legs of the antihelix, As such, people typically need to tug on the shell in order to pull it out, much like how you tug on an earplug.