Miley and Lorde are no where near Gaga's level. Let's not even go there. So what, they had some hits. That doesn't make them a "bigger popstar" than Gaga. More relevant right now? Probably. But, uh, it's not even worth explaining.
Gaga isn't the biggest popstar right now, that's Katy, but she's certainly bigger than Miley and Lorde.