Was this an oversight or intentional?
In general, damage scales 1 die (or more) with each increase in size:
Berserker (Medium): Greatclub. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 7 (1d8+3) bludgeoning damage. (instead of Greataxe)
Ogre (Large): Greatclub. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage.
Hill Giant (Huge): Greatclub. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 18 (3d8 + 5) bludgeoning damage.
Now dragons (Red):
Wyrming (Medium): Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 9 (1d10 + 4) piercing damage plus 3 (1d6) fire damage.
Young (Large): Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 6) piercing damage plus 3 (1d6) fire damage.
Adult (Huge): Bite. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 19 (2d10 + 8) piercing damage plus 7 (2d6) fire damage.
Ancient (Gargantuan): Bite. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) piercing damage plus 14 (4d6) fire damage.
So... why don't Adult dragons do 3d10 and Ancient dragons do 4d10??? An ancient dragon doing only an average of 4 damage more than a young dragon seems very, well, wrong.
EDIT: claws are also an issue.