It was huge because it was a) an extremely catchy song, b) the song (both sound and lyrical content) and music video (both visuals and looks) were safe, and c) released right at Katy Perry's peak as a pop star.
It had great promotion (hello, it was even performed in front of 120 million people at the Super Bowl) and the music video went viral.
I hope this helps you understand its success better! Can't wait for the song to go Diamond.
