I'm afraid that isn't true - apart from minor local strengthening to cater for the extra MTOW the A320 and A321 wings are identical.
The green dot speed is the speed at which one gets best lift/drag ratio (clean aircraft).
Best L/D is at a speed where zero lift (profile) drag equals drag due to lift.
The A321 fuselage is 7m longer than the A320, leading to a higher value of profile drag and therefore a higher lift dependent drag for best L/D. (Actual value of the best L/D will of course be lower on the A321). This in turn means that green dot conditions are found at a higher lift coefficient on the A321 than the A320 so that with the same wing and at the same weight the green dot speeds will differ.