Google's Docs and Sheets apps were both published on the Play Store in April 2014. Docs hit 500 million Play Store installs in October of last year, and it's now Sheets' turn. By comparison, Google Slides is still at the 100 million mark, though we wouldn't be shocked to see it reach 500 million soon as well.
Given that people are more likely to use a word processor on their phones and tablets than a spreadsheet editor, it's no surprise that it took Sheets a few extra months to hit the same 500 million mark as Docs. Any bets on how long it'll take Slides to reach the same milestone?