Starglim |
According to the PRD, brass knuckles are an unarmed attack and not a light, one-handed or two-handed weapon. It looks to me as if they can only be used as a weapon (with proficiency) to enhance a punch, which is usually a light weapon for a creature. It's possible a creature one size smaller could use them as a one-handed weapon, or two sizes smaller could swing them two-handed.
Otherwise they would just be an improvised weapon, using their weight alone for the appropriately scaled amount of damage.