The heavyweight champion of the world earns the unofficial designation of Baddest Man on the Planet, a title no one at first glance would confer to Andy Ruiz Jr. He carries, in his own words, “extra flab.” His muscles are undefined, and his belly jiggles. His opponents evoke statues of Greek gods. He evokes a man ordering an extra gyro.

“A lot of people can relate to me,” Ruiz said Tuesday in a phone conversation. “It doesn’t matter how you look. As long as you train hard, you focus and you’re hungry and that drive is in you to follow your dreams, everything is possible.”

Ruiz has been overlooked, mocked and doubted his entire boxing career, judged on his appearance rather than his quick hands and iron fortitude. No more. Ruiz gained instant fame and long-sought validation Saturday night at Madison Square Garden, where in the seventh round he knocked out previously undefeated champion Anthony Joshua, a Brit four inches taller than Ruiz and built like Adonis. Ruiz was an 11-1 long shot, and the victory entered the pantheon of biggest surprises in boxing history.

Ruiz received his title shot only one month before the fight, a late replacement after Jarrell Miller, a Brooklyn fighter chosen to provide a hometown crowd favorite, failed a third performance-enhancing drug test. Eddie Hearn, Joshua’s promoter, first considered Ruiz as a fill-in after Ruiz sent Hearn an Instagram direct message pleading for the chance, insisting he was the best contender available.

“I knew if I would take this fight, I was going to win,” Ruiz said. “And that’s what we did. We shocked the world.”

A 29-year-old from Imperial, Calif., Ruiz became the first heavyweight champion of Mexican descent and shook up the heavyweight division, raising questions about whether Joshua still belonged in the same class as Tyson Fury and Deontay Wilder while adding a major draw to the division.

But what has attracted and delighted casual fans is Ruiz’s doughy figure. Ruiz is a skilled, tough fighter in the package of an everyman. He is 6-foot-2 and weighed in at 268 pounds. He was a self-described “chubby, nice kid” who never grew out of his chubbiness or niceness.

At times, Ruiz admitted, he struggled with people judging him for how he looked. “I’ve been bullied a few times in school,” Ruiz said. Manny Robles, Ruiz’s trainer, has discussed some of those moments with Ruiz.

“It’s a Cinderella story,” Robles said. “This is for all the people out there who get harassed and pushed around. These kids nowadays, the society we live in, kids being bullied and being harassed, this is for them. This is for all the people to believe and to know that anything in life is possible.

“The great majority of us people are Andy or look like Andy. Everybody wishes they had a body like Anthony Joshua. We just don’t.”

On Saturday night, Ruiz entered the ring with the nickname “Destroyer” displayed across the front of his trunks. It is a typical moniker with an atypical origin: As a rambunctious child, he would break his mother’s things and smash new toys immediately after he received them.

His hyperactivity convinced Ruiz’s father, Andres Sr., to drop him off at a boxing gym at age 6. The sport ran in the family: Ruiz’s grandfather trained fighters in Mexicali. He sparred with older fighters, got beat up and wanted to quit. Andres Sr. encouraged him to stay, telling him, “You’re going to end up beating everyone up.” At age 7, Ruiz fought his first amateur bout in San Diego.

Ruiz grew up in Imperial, a small town about 12 miles north of the Mexican border. His parents live a few blocks from Imperial High, which Ruiz attended before leaving to train full time in Mexico. The football coach still unlocks the weight room for Ruiz early in the morning when he is back home.

“Everybody knows everybody here,” Imperial High Athletic Director Victor Cruz said. “We all know who Andy is.”

Cruz was reluctant to say much about Ruiz because he has been disappointed by media coverage focused on the fighter’s pudginess. “A lot of people have been mocking Andy’s physique,” Cruz said. “It doesn’t reflect what kind of athlete he is.” Cruz described Ruiz as a “happy giant” during his high school days, always smiling and joking.

“Nobody was going to give Andy a hard time about his size, because people are aware of what Andy can do,” Cruz said. “Nobody was going to mess with Andy.”

In 2008, Ruiz qualified for the Mexican Olympic team. “That’s when I gave myself more confidence and I knew what I could accomplish in my life,” he said.

Ruiz entered the Joshua fight with a 32-1 record, and even though he had collected 22 knockouts, he carried a reputation as a fighter with lightning-quick hands and an iron jaw but one with little power for a heavyweight. He could take a big punch, but he couldn’t deliver one.

“He’s been in with guys so much worse than Joshua who he couldn’t really hurt at all,” said promoter Bob Arum, who promoted Ruiz for all but his past two fights. “When you look at his record, considering the guys we matched him with, he didn’t have that many knockouts.

“He has very good hands and a tremendous chin. You could hit him with a baseball bat, and nothing happens. That part didn’t surprise me. What surprised me was how he was able to knock Joshua down as frequently as he did. He wasn’t able to do it to anybody else of any quality.”

Ruiz actually gained five pounds since his last fight to face Joshua, wanting the extra power against a taller boxer. For his standards, Ruiz was in good shape.

“If you go back a few fights, he was much bigger than what he was Saturday night,” Robles said. “I want to say he was in great shape for this fight Saturday night. He physically looked his best.”

For legions of fans tuning in to see Joshua’s first appearance in America, the physical contrast was jarring. Joshua was one of boxing’s biggest stars, a hulking, chiseled specimen who had fought in front of crowds of 90,000 in England.

“Looks can be deceiving,” Robles said. “You look at Andy, and you might think he’s not much of a fighter. But once you get that boy in that ring, man, believe me: He’s just a different animal. … He’s not scared of anything. He’s not scared of anyone.”

Joshua knocked down Ruiz in the third round, the first time Ruiz had hit the canvas in his entire career. When Ruiz rose, Robles thought, “Oh, my goodness, Joshua’s in trouble now.” Ruiz conjured “the Mexican warrior that I have inside me,” he said, and knocked down Joshua before the round ended. He knocked him down thrice more, and in the seventh, the referee ended the fight after Joshua refused to move forward.

“Everybody is looking for that chiseled guy,” Ruiz said. “Just because they have a lot of muscles, they think he’s the strongest guy out there, this and that. Lucky for me, the skills pay the bills.”

Ruiz celebrated with friends and family at a New York steakhouse, and by the time he met Robles at the airport the next morning, neither man had slept. “I still can’t believe this, Manny,” Ruiz told his trainer. “I got to be pinching myself.” On the flight home, he reminisced about difficult times, thinking about the things his family wouldn’t have to want anymore.

Ruiz intends on keeping his three championship belts, starting with a rematch against Joshua. He feels new motivation to improve his physical condition. In his news conference Saturday night, he declared a new goal: “I want to get in really good shape and look like a Mexican Anthony.”

He understands it will be difficult. Asked which foods he will have to stop eating as part of his new regimen, he replied: “A favorite food? I have too many.”