He's bigger than Conor. He's stronger than Conor ... but if Jason Momoa ever got in a fight with Conor McGregor -- IT WOULD BE LIGHTS OUT, so says UFC's Bruce Buffer. 

"Muscles don't win fights," Buffer told us when we asked how the 6'4", 230-poundish musclebound Momoa would do inside the Octagon. 

Don't get us wrong, Buffer says he's got all sorts of respect for the "Aquaman" actor -- but even the most ripped, beefed up dudes would get SMASHED by the smallest pros in the UFC. 

"I'd put a 135 pounder or a 145 pounder like Conor McGregor on him and it'll be a whole new world for him because he's not a fighter."

"He'd get [Momoa] on his back, he'd choke him out. You put him in an armbar, the biggest guy in the world is gonna tap or cry his eyes out as his arm gets broken."

Buffer says Momoa has the size and toughness to be a decent fighter if he wanted to ... but bottom line -- don't ever confuse muscles for fighting ability.