Maybe 19 can reach 4 million copies sold once the reign of 25 is over? Got a while until this era ends with the inevitible 2017 grammy sweep for Adele still to come. 21 should pass 13 million by the end of the era too.
I can see 25 going diamond for sure though. Going to be an amazing ride.
Usually when someone says "biggest since _______" it implies that that person was bigger, otherwise they would have said "biggest since..." someone else who actually was bigger.