Shakira has literally the longest career and still slaying a lot of Pop Girls who came later, her talent and Global appeal is undeniable so her career is bigger Britney is close too since she was never local and her success is huge.
Lady Gaga, Rihanna, Katy Perry, Adele, Beyoncé, Shakira (the only latin artist the can compete with the pop girls)
Gaga is the only artist that has two albums over 7 million this decade (Born This Way and The Fame Monster sold more than 7 million this decade), maybe Adele but I'm not sure how much of 19's sales are from this decade. They did numbers no other pop girls have done this decade.
The amount of huge smash hits Katy and Rihanna have is quite impressive and their album sales are great too!