Must a PC (or other creature) be Huge in size to deal the 3d12 damage dice upon a hit with a Huge-sized greataxe as per the oversized-weapon rules?
I believe the oversized-weapon rules are somewhat ambiguous as to whether the extra damage dice are a property of the size of the weapon (by itself) or result only when a big weapon is wielded by a similarly big creature. The text at issue is in the DMG under the chapter entitled "Creating a Monster"->subchapter "Creating a Monster Stat Block"->part "Step 11. Damage"->subpart "Base the Damage on the Weapon":
Big monsters typically wield oversized weapons that deal extra dice of damage on a hit.
This sentence suggests that the extra damage is a property of the weapon itself, independent of the size of the monster wielding it.
Double the weapon dice if the creature is Large, triple the weapon dice if it's Huge, and quadruple the weapon dice if it's Gargantuan. For example, a Huge giant wielding an appropriately sized greataxe deals 3d12 slashing damage (plus its Strength bonus), instead of the normal 1d12.
Here, the first sentence implies that any creature can use the oversized weapon, but it also seemingly links the number of damage dice to the size of the wielding creature instead of to the weapon itself. The example also specifies that both the greataxe and the giant are Huge sized. Still, especially given the next paragraph, by referring to the creature's size, the first sentence might best be read as referring to the weapon via the "appropriately sized" creature--thereby essentially categorizing the weapon itself as Large, Huge, or Gargantuan.
A creature has disadvantage on attack rolls with a weapon that is sized for a larger attacker.
This confirms that other creatures may wield the oversized weapon, but it does not clarify whether a successful attack would result in the additional weapon damage dice.
It looks like similar topics have been discussed before, but I didn't see anything directly answering the question.