The gap between Tove's album and the release of Habits is around the same as Zara's album and the release of Lush Life so it's not like she's any better
And Habits only doing 1m more than Lush Life despite the latter going top 5 in the US(the biggest market in the world) is not something to be proud about, especially with the 2014 sales climate being way better than what we have now
But still, you're comparing Zara in 2016 (Lush Life didn't smash outside of Scandinavia until 2016) to Tove in 2014, 2015, and 2016
That alone should tell you that even if Zara isn't bigger now, she will be in the future