Meh, Rihanna and Katy aren't too far apart.
Albums since 2008 (Katy's debut)
Rihanna - 3.7M (2008), 2.2M (2010), 3.4M (2011), 3.2M (2012), 1.9M (2013) = 14.4M with 1 re-release and 4 studio albums.
Katy - 1.5M (2008), 1.3 (2009), 2.2M (2010), 2.1M (2011), 1.4M (2012), 1.8M (2013), 1.2M (2014) = 11.5M with 1 re-release and 3 albums. Assuming Katy had 1 more release during that span, she'd probably be the same.
Tour gross (2010 & forward since they both kinda had flop tours in 2009):
Rihanna: LOUD (90M) & Diamonds (140M) = $230M
Katy: California Dreams (60M). Prismatic (IDK she has 2 legs left at least, I'll say 150M) = $210M
$30M difference isn't a lot. And if Prismatic does $180M or more, they're the same.
And as far as singles go (2010 & forward)...
Rihanna - 8 #1's (6 solo)
Katy - 8 #1's (all solo)