Happy sold a million more than Dark Horse in the US alone (probably more since some of Dark Horse's sales were before the tracking period), and at least a million more in the UK than Dark Horse too (since Happy is at 2 million units while Dark Horse isn't at a million yet). So that's a 2 million lead Happy has at the very least thanks to those markets, yet on the IFPI chart it's only 700k ahead worldwide. Dark Horse must have been pretty global outside those 2 markets!
who in their right mind would say that Dark Horse is the biggest hit song by a woman of all time?
I think of I Will Always Love You, My Heart Will Go On, All I Want for Christmas is You for physical singles
Call Me Maybe, Rolling in the Deep, Tik Tok, Poker Face, and even Firework were bigger hit songs in the digital era