How about a magic item?
Giant Bone Brass Knuckles
weapon (brass knuckles), rare
These +1 brass knuckles are not made of brass, but the hollowed and carved bone of a giant. Any creature who wares this item deal damage as if their unarmed attack did 1d4 damage or their unarmored attack, whatever is higher, becoming proficient with the attack as well as well as the bonus from the enchantment. In addition, up to three times per day, when a creature is touched by this weapon, the wearer can activate the spell enlarge/reduce on the target. The wearer can punch their own fist and activate this on themselves if they wish. The DC for this save is equal to the spell save DC of the wearer or 8 + wearer’s proficiency + Wisdom modifier.
Thoughts?
Throat Punch Games 