How are you coming up with 4d6+5? The part you quoted listed the damage as 2d6 + mod. There was nothing that said this should be doubled. I believe the whole point of the 2d6 is that is the Large damage from the Medium damage of 1d6. Why would you every double that again?
They can now use large weapons.
Large weapons deal double weapon dice damage. So our 8th level troll wields a large great sword which deals 4d6 base damage.