In D&D 5e, according to p. 278 of the Dungeon Master's Guide, if a Medium-sized character uses a Large weapon, its damage dice are doubled, but it causes them to suffer disadvantage on their attack roll:
Big monsters typically wield oversized weapons that deal extra dice of damage on a hit. 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.
A creature has disadvantage on attack rolls with a weapon that is sized for a larger attacker. You can rule that a weapon sized for an attacker two or more sizes larger is too big for the creature to use at all.
The answers to the question Do weapons looted from creatures that are larger than Medium in size retain their damage when wielded by Medium-sized PCs? support this interpretation. However, my question is not asking whether wielding oversized weapons is allowed or not.
Assuming that characters are allowed to wield Large-sized weapons, when (if ever) is it statistically advantageous to do so?
Let's take the example of a Variant Human Barbarian with a starting Strength score of 16, and Great Weapon Master as their racial feat.
- Are they better off using a Medium or Large Greataxe?
- What about when Reckless Attack, Rage and/or Great Weapon Master
enter the mix?
I expect that this would probably depend on the AC of the target, but how would they compare?
- Does this change as the Barbarian levels up and gains additional damage from their Strength modifier or Rage bonus increasing?