From Teflpedia

A diminutive (/dəmɪnjətɪv/) is a phrase (often a single-word phrase) that has been modified to convey a slighter degree of its root meaning, to convey the smallness of the object or quality named, or to convey a sense of intimacy or endearment.[1]

This is done in two ways; (1) by using an adjective, especially "little", e.g. "Little Mix", "Tiny Tim", or (2) by using a diminutive suffix[2]