Okay, say a character used the runic juggernaut ability from Rune Knight to become Huge. Now normally this just increases the characters attack damage by 1d10 once. But what if they were fighting a fire giant, the giant was disarmed and then picked up their weapon. A Fire Giants greatsword does 6d6+str slashing damage, the damage shouldn't decrease if wielded by another huge character. If I'm reading this correctly the first attack in a round a rune knight makes with a Fire Giants greatsword actually does an extra 1d10 damage on top of the 6d6+str with the first attack. Now obviously this is op, but a Fire Giants Greatsword weighs 384lb's (6lb's x 8 x 8) so it's not as if a party member can carry it around with them when they become medium again... unless that character has Powerful Build...
The only thing I could find on whether this would be allowed is this sentence from the DMG (Chapter 9, Creating a Monster Stat Block, Step 11.) "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." It doesn't specifically mention anything such as "A medium creature under the effect of an enlarge spell does not gain disadvantage when attacking with a weapon sized for a large creature (such as an Ogres Club)". Also, according to this, if you ever want a comedy style enemy, you could have an ogre who stole a Giants sword who always attacks with disadvantage cause the sword is too heavy for them.
TL;DR Can a creature, whilst under an effect that increases their size (Giants Might, Enlarge/Reduce, be a Werebear etc) wield weapons designed for larger creatures without having disadvantage or reduced damage
Okay, say a character used the runic juggernaut ability from Rune Knight to become Huge. Now normally this just increases the characters attack damage by 1d10 once. But what if they were fighting a fire giant, the giant was disarmed and then picked up their weapon. A Fire Giants greatsword does 6d6+str slashing damage, the damage shouldn't decrease if wielded by another huge character. If I'm reading this correctly the first attack in a round a rune knight makes with a Fire Giants greatsword actually does an extra 1d10 damage on top of the 6d6+str with the first attack. Now obviously this is op, but a Fire Giants Greatsword weighs 384lb's (6lb's x 8 x 8) so it's not as if a party member can carry it around with them when they become medium again... unless that character has Powerful Build...
The only thing I could find on whether this would be allowed is this sentence from the DMG (Chapter 9, Creating a Monster Stat Block, Step 11.) "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." It doesn't specifically mention anything such as "A medium creature under the effect of an enlarge spell does not gain disadvantage when attacking with a weapon sized for a large creature (such as an Ogres Club)". Also, according to this, if you ever want a comedy style enemy, you could have an ogre who stole a Giants sword who always attacks with disadvantage cause the sword is too heavy for them.
TL;DR Can a creature, whilst under an effect that increases their size (Giants Might, Enlarge/Reduce, be a Werebear etc) wield weapons designed for larger creatures without having disadvantage or reduced damage
So, RAW it does the damage of a normal great sword plus any bonus damage.
The DM may choose to use the damage of the creature stat block (you probably wouldn't do bonus damage then).