Because Rihanna debuted at such an early age the comparison is unfair. She should be compared to girls who debuted when they were younger like Taylor Swift.
Regardless, both Rihanna and Katy will be remembered from the late 2000s and 2010s because they have the ability to collect hit after hit.
No especially when the public is already over Katy her cotton candy persona... which explains why 60% of the singles from Prism flopped
Her videos sans Birthday have lots of views for flop singles, even surpassing the views for other girls' lead singles so she is doing fine. She might not be bigger than Rihanna but all her singles have hit the top 40.
Katy has sold around 14 million with only 3 albums which is 4,3 million on average.
Rihanna has sold 30 million with 7 albums, or 4,29 million on average.