She's not tho
C&GI has outsold TW, just hit number 1 on pop, and she's about to go on a pretty big world tour. The only Disney girl she MIGHT be outperforming would be Demi, but even then... Lets see how Ariana's, and the other Disney girls albums go
EDIT: I mean don't get me wrong TW has been extremely successful, and has probably exceeded everyone's expectations, plus it's a great song, but to say she's out performing Selena and Miley would be stretching things a bit to far