The 39-year-old Pittsburgh Steelers QB has looked much slimmer leading into the new season ... with folks left wondering what the did to get in better shape.

Big Ben -- who's always been listed at 240 lbs. -- is finally speaking out on his trimmed look ... but as it turns out, he's denying he's doing anything differently from years past.

"So much was made a week or 2 [ago]," Roethlisberger told the Washington Post. "I mean, listen, as you get older ... for the last 4 or 5 years, I’ve really focused on having a trainer, my body."

"Part of it’s football. But part of it’s life. I’m not getting any younger. I’m almost 40 years old. So you’ve got to take care of yourself."

But, Ben makes it clear he's not taking any tips from his 7-time Super Bowl champion rival ... saying there are some levels he simply will not stoop to in order to stay in shape.

"And nothing against Tom [Brady], but I’m not eating avocado ice cream," Roethlisberger added. "Yeah, you have a chef and you try and eat healthy. But you’re eating healthy because you want to be and feel good."

Remember, TB12 has sworn up and down in the past that the dessert is tasty ... and even included the recipe -- consisting of avocado, cashews, coconut, dates, cacao powder and water -- in his "The TB12 Method" book.

But we spoke with Brady's bestie, Julian Edelman, in the past ... who confirmed to us it's really as bad as it sounds.