W0LFB3AT5 said:
Then what’s the opposite of medium?

Don't know if it's recognized the same all over the world, but in the college level/above-college level Japanese linguistic sphere, "opposites" (in any language) are characterized as being able to fall into one of three determined groups. One type expresses "if not A then definitely B" (such as "man/woman," "adult/child,"). One expresses "an action or thing perceived as being in opposition" ("buy/sell," "come/go," "peace treaty/declaration of war"). And the final type expresses "opposing levels of a spectrum" ("long/short," "high/low").

Since "medium" expresses the middle point of a spectrum, it can't have an opposite word in the final group. And as it's an adjective and not an action or noun, the second group is also impossible. But the first group remains possible, and the corresponding "opposite word" would be said to be something along the lines of "irregular size(d)."

this is how it is in english too, i think. i'm actually learning about this in a semantics/pragmatics class right now
the first type you mention above is called "complementary antonyms" (can only be one or the other)
second would probably be closest to "reverse antonyms" (denoting change or motion in one direction or the other)
and the third is "gradable antonyms", with the two words being extremes on a spectrum (with any number of degrees in between)
funnily enough i just took an exam over this subject matter earlier today