John Mayer on Katy Perry's 'Roar': 'You Get Swallowed Up By It, It's So Big'

John Mayer knew from the moment he first heard his girlfriend Katy Perry's new song, "Roar," that it would be a "megaton bomb"-sized summer hit. The singer-guitarist says he got wrapped up in the moment of witnessing the creation of a "huge song" before it even "left the stable."

"I remember hearing it, going like, 'Wow what an interesting experience' to hear something that you identify as massive before it's massive," he tells Billboard. "I thought it was a really sweet, unforgettable sort of experience to hear someone's work while they still have a very nascent relationship with it."

Listen to Katy Perry's 'Roar'

Perry's "Roar" leaked to the Internet early on Saturday (Aug. 10) and was quickly serviced to radio stations later that day. The track reached digital retailers at 12 a.m. on Monday (Aug. 12) and shot to No. 34 on the Pop Songs radio airplay chart.

"In a way you get swallowed up by it, it's so big," he says, adding: "Because it's such an incredibly big song that it doesn't need you to tell anybody, while you're eating lunch, [that] millions of people are going to be dancing on tables to that."

Coincidentally, not long after "Roar" sprung a leak, Mayer experienced his own breach when "Who You Love," a duet with Perry that appears on his "Paradise Valley" album (out Aug. 20), made its way to the Internet. In his chat with Billboard, Mayer says he wanted to write a melodic, and confessional, song about accepting who you love.

LISTEN: Mayer and Perry's 'Who You Love' Hits Web

"I started singing 'And my girl, she ain't the one I saw coming / Sometimes I don't know which way to go /And I tried to run before, but I'm not running anymore.' In 2013, modern-day times that's sweet. Because there's so many other things to occupy your time that you can do for yourself. That to say, I tried to be alone but I can't. This is sort of our generation."

Mayer says the idea of the gentle, breezy song could be described as "I love you based on the fact that I've tried to run and I'm not running and I give up." He says Perry wrote her contribution, a verse admitting "my boy was not the one that I saw coming, some have said his heart's too hard to hold."

"I immediately thought about Katy and I was like, what a cool voice. What a cool artistic personality to have on. It was a really fun opportunity for her to write like, her answer to that. So you get these two sides to this relationship that are brutally honest but no less universal."