Does "biggest" here mean "greatest"? Because I think BTS is not the largest group with the most members.
You are almost correct! "Biggest" in this context means "extremely popular," "important" or "well-known." Ellen uses "biggest" so that her audience can grasp the idea of just how popular BTS is all over the world. Ex: He is the biggest star in the world right now. Ex: Michael Jordan was one of the biggest basketball players for the Chicago Bulls. He is still popular to this day.