Jameis Winston -- 'HE'S NOT THAT FAT' ... Says Famous Fat QB

STOP BODY-SHAMING JAMEIS WINSTON ... he's got a perfect QB physique ... so says one of the most famous heavyset NFL quarterbacks of all time ... former NY Giants QB Jared Lorenzen.

Of course, Winston has been under fire from people who say he looks too fat, doughy, soft and out-of-shape to be a successful NFL QB ... especially after he went topless at his Pro Day this week (see pic above).

But Lorenzen -- aka "The Hefty Lefty" & "The Pillsbury Throwboy" -- says Winston needs to ignore the haters and focus on winning football games.

"He's not JaMarcus Russell. He's not fat by any means. He's just a big dude," Jared says. "He's got a great body to play quarterback. The little skinny guys don't make it. They can't take a hit."

"Look at RG3 ... small guy. Now he can't even run."

BTW -- when Lorenzen played for the Giants from 2004 to 2007 he usually hovered around 300 pounds.

"I was probably a little bit on the extreme side ... but I never missed a game due to injury in my career going all the way back to when I was young. Most big guys don't. They can take a pounding."

As for Jameis, Jared says the guy's potential is off the charts -- "He's only 21 and he doesn't even have his grown man body yet. He's gonna get bigger and faster."

BONUS -- Here's footage of Jared playing for the Northern Kentucky River Monsters last year. He's big ... but the dude can still ball.