bantam
Small but very bold and full of fighting spirit.
Bantam refers to something small but spirited and scrappy. The word comes from a breed of small chickens that, despite their size, strut around the barnyard with remarkable confidence and can fight fiercely when challenged.
You'll often hear bantam describe athletes or fighters who compete in smaller weight classes. A bantamweight boxer might weigh only about 118 pounds, but fights with the same intensity as much heavier competitors. In hockey, a bantam league is for players around 13-14 years old.
The word captures a particular quality: being small but refusing to act small. A bantam rooster might be half the size of other chickens but acts like he owns the whole farm. When someone calls a short person a bantam, they're usually noting that small size combined with outsized confidence or fighting spirit.
You might describe a scrappy point guard who fearlessly drives between much taller players as having a bantam attitude. The word suggests that size doesn't determine courage, determination, or force of personality.