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PHILADELPHIA — “Honestly, we talked about just doing one show this year.”

Kenny Chesney chuckles and shakes his head. The country music superstar is sitting on his tour bus parked at Lincoln Financial Field, home to the Philadelphia Eagles, trying to explain how his “Brothers of the Sun” tour with Tim McGraw got a little, well, a lot bigger than he expected, as 53,000 folks enjoy a lively afternoon tailgate outside on a scorching June afternoon.

“I called him and said, ‘Look, I’m not sure what I’m doing next year. I know I want to do at least one stadium show. What do you think about doing it with me?’ ”

McGraw, a longtime Chesney friend, a country superstar in his own right with a new album — “Emotional Traffic” — and no dummy, said yes. And then, says Chesney with a laugh, “One show led to two. And then everybody started talking and said, ‘If we’re all good with doing two, why can’t we do eight?’ And then next thing you know it’s 16. And then it’s 23. And we’re doing two in Boston again.”

It’s not just the number of shows that’s big. According to Pollstar, “Brothers of the Sun” is the highest grossing tour of the summer, raking in more than the next artists on the list — Coldplay and Roger Waters — by a roughly 2-to-1 margin and selling over a million tickets. It comes to a close with shows this Friday and Saturday at Chesney’s local home away from home, Gillette Stadium.

In his dressing room, McGraw contemplates his and Chesney’s long friendship and the ways things have changed since they last toured together in the early ’00s.

“There was a lot of drinking involved before,” says McGraw with a chuckle, noting he hasn’t had a drink in four years.

The pair clearly don’t need alcohol to throw a party, as will be evidenced later in the evening when McGraw strides out during Chesney’s encore to trade songs with his buddy, as well as openers Jake Owen and Grace Potter & the Nocturnals. The bond between the pair is clear both onstage, especially during their revved-up recent hit duet “Feel Like a Rock Star,” and off, when the two talk about each other and the ways in which the landscape has shifted for them in the intervening years.

McGraw, the headliner earlier on, became even more popular with a string of hits including “Live Like You Were Dying,” “Southern Voice,” and the recent “Felt Good on My Lips.” He also embarked on a successful acting career in films like “Friday Night Lights” and “The Blind Side.”

And Chesney simply exploded, scoring his own clutch of hits dedicated to nostalgia (“I Go Back,” “Young,”) and the escapist, laid-back Caribbean/country lifestyle (“When the Sun Goes Down,” “Beer in Mexico”) that catapulted him to the top of the touring heap. Combined they have more than 50 No. 1 singles and trophy cases groaning with hardware from the Grammys, the Academy of Country Music, and the Country Music Association, among others.

“Me and Tim have been friends a long time. We’ve been on different paths. We’ve been on parallel paths. Our lives have changed in a lot of ways since the last time that I opened for him in 2001. But the moment that he comes up onstage with me is a great moment,” says Chesney. “He doesn’t have to be there first of all, and he is. There’s years of investment with us.”

Chesney and McGraw also find themselves at similar places in their careers and enjoying the sense of creative liberation that comes with success.

“I feel like a young artist and I’m just really starting to figure out who I am and what it is that I do and just starting to get halfway decent at it,” says McGraw of the varied sounds of “Emotional Traffic.” “You try to grow. I want to get better. If I didn’t think I could get any better I’d probably quit doing it.”

For his part, Chesney continues to expand his horizons as a songwriter and in song choices and trusts that his audience will be open to the good-time anthems as well as more reflective songs like “You and Tequila” — featuring Potter — from 2010’s “Hemingway’s Whiskey” and tunes like “I’m A Small Town” on his latest album, “Welcome to the Fishbowl.”

“When you get to the point in your career that I have, you start competing with yourself in ways, and I do feel like I need to push my audience but not alienate them,” he says. “That is a small target to hit, but it’s possible. It’s a great creative place to be in right now.”

As is the big stage they share.

“It’s an incredible feeling to have that many people at one time all on the same wavelength,” says McGraw. “It’s something that you live for.”