As if to prove that point, Chamberlain unexpectedly leans back in the folding chair that sits by his locker, pulling up the bottom of his team-issued sweatshirt and exposing a stomach that is indeed flatter than you might expect.
There is no argument that Chamberlain appears bigger, but he said the extra bulk is the end result of a winter working out in the basement of his Lincoln, Neb., home, where he constructed a personal gym to get a head start for the season.
"Obviously, I added muscle," Chamberlain said. "That just comes from being a man, too. I'm 25 now, not 21. That's part of it, and it's not a huge difference. Physically, I feel so much better. My recovery is better, and I think that's because I started so much sooner and understand my body a little bit more."
Chamberlain's weight became the talk of Yankees camp on Wednesday, as general manager Brian Cashman acknowledged that there were additional pounds on Chamberlain's 6-foot-2 frame. He officially remains listed at 230 pounds.
"He's heavier -- let's just leave it at that," Cashman said. "Anything else, we'll deal with. He works hard. He's a good kid. He's heavy."
Cashman said that Chamberlain is not the only player who reported over the prescribed weight -- non-roster invitee Bartolo Colon is laughably listed at 185 pounds by the club. Manager Joe Girardi said that he noticed Chamberlain's shaggier haircut more than any other change.
"I thought he looked a lot like he did last year," Girardi said. "I think his hairstyle being different is the one thing I noticed first. Joba is a big, barrel-chested guy anyway. He's always going to look big."
In an interview with MLB.com, Chamberlain said that he decided to invest in the gym after last season, partly to combat his commute during the frigid Nebraska winter.
"I wanted to have that little edge of knowing that when there are days when you don't want to go to the gym, you know all you've got to do is go downstairs instead of getting in your car," Chamberlain said.
Chamberlain consulted with Dana Cavalea, the team's director of strength and conditioning, and came up with an order that includes a treadmill, dumbbells, a Keiser trainer and a Power Plate vibration machine.
"It's everything I need," said Chamberlain, who invited his 5-year-old son Karter to watch some workouts. "It's not overly huge, but it's enough to where I can have everything and have some room to work out."
Chamberlain lost his setup role last season due to inconsistency, finishing 3-4 with a 4.40 ERA in 73 appearances. But he was motivated by the second half of the season, posting a 2.88 ERA in 34 outings and limiting hitters to a .220 average.
"Looking back at the second half of last season, I started lifting hard toward the All-Star break, and I had a great second half," Chamberlain said. "I felt instead of coming into Spring Training and starting to lift heavy and doing things like that, I thought I would start earlier and see how it works."
Chamberlain said that the results have been great. He threw a bullpen session on Wednesday with catcher Russell Martin, and Girardi said that Chamberlain has had no issues keeping up with the pitchers during exercises.
"Joba's gone through all of the drills fine," Girardi said. "I watched him run yesterday; he was fine. I've watched him throw. I've been very happy with the way he's throwing the baseball.
"Weight can be a tricky issue sometimes. You're evaluated on how you pitch, and a lot of times, if you're not pitching well, they'll say you're too heavy or you're too light. If you're pitching well, no one says a word. It's the same guy."
Girardi noted that Chamberlain seems to be a focus every season, and this weight gain will be another note in a four-year rush that has seen him go from reliever to starter and back again.
Even though the Yankees are searching for their fourth and fifth starters, Chamberlain will not be considered, instead competing with Dave Robertson to pitch the seventh inning behind Rafael Soriano and Mariano Rivera.
Why not try Chamberlain alongside the likes of Colon, Sergio Mitre, Freddy Garcia and Ivan Nova? Cashman said that Chamberlain's velocity as a starter was affected by an August 2008 shoulder injury.
"It used to be the same out of the rotation and the bullpen -- it played the same," Cashman said. "Now, when you scout him, it's not. It's radically different out of the bullpen.
"I get a kick out of when people [say], 'Oh, they're so indecisive. They don't know what to do with him.' No. The stuff used to be equal. It's not equal anymore."
Chamberlain said that he can still throw as hard as he used to, but the right-hander did allow that he may have compensated for the shoulder injury. He is hoping that moving his hands lower, where they were in 2007, will be an easy mechanical fix.
"Your mechanics dictate so much," Chamberlain said. "When you try to compensate for something, you're just trying to play catch-up. There were times I'd be there and times I wouldn't. You've just got to minimize those periods where you can't figure it out."
Bryan Hoch is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.