You can hold one picture for this whole word: someone about as tall as a fist. The Greek root really is the word for a fist, and the old stories used it for a people that small. Every later use is that picture moved somewhere new — onto real people, onto a kind of animal, and onto how much a person counts in their field. Where the picture lands matters a great deal, though. On an animal name it is a neutral size label. On a person it is either a dated term that people object to, or a straight insult.
Recorded as coming from Latin Pygmaeī, from Ancient Greek pugmaîos, a member of a legendary race of dwarfs, built on pugmḗ, a fist — that is, as tall as a fist. The sources also note it was first used of actual living peoples only in the 19th century.
來源記載它來自拉丁文 Pygmaeī，再往前是古希臘文 πυγμαῖος（傳說中小矮人族的一員），而這個字建立在 πυγμή（拳頭）之上，也就是「只有一個拳頭那麼高」。來源同時註明，一直到十九世紀，這個字才被拿去指真實存在的族群。
Wiktionary (kaikki extract, source_ref kaikki:3f6f69a190036ae4:590088527)
Three branches
小個子的人
這是核心畫面本身：身高只有一個拳頭的人。家族裡的兩個意思，差別在於講的是傳說，還是真實存在的族群。
noun · C1
In old Greek stories and in folklore, a member of a race of very small people. This is the oldest meaning of the word, and it is still the one used when people are writing about myths.
（神話中的）小矮人族
小型的動植物
同一個「小得出奇」的畫面，貼到生物的名稱上。這時它是尺寸標示，不是評價，所以不帶貶意。
adjective · B2
Used in front of the name of an animal or plant to show that this kind is much smaller than the other kinds of the same animal or plant.
（動植物）小型的
分量很輕的人（比喻）
畫面從身高搬到地位：同一行的人排成一排，這個人只到別人的膝蓋。這是刻意的比喻，不是在描述體型。
noun · C1
Someone who counts for very little next to others doing the same thing — much less able, less important or less powerful than the rest. The word shows that the speaker thinks little of them, and it is an insult.
無足輕重的人