The mma fighter had the advantage the longer the fight dragged on, sumo is a sprint compared to MMA. If the sumo could have pinned him down early in the match and wailed on him he could have won, but the MMA fighter had every advantage other than weight and strength.
Stamina, speed, experience. There's a lot to being trained how to grapple and properly throw punches and kicks. You don't need a tons of strength to knock someone out, it's actually more about hitting just the right spot.
That was a mismatch from the beginning, the sumo guy had little chance
"Suddenly Frodo noticed that a strange-looking weather-beaten man, sitting in the shadows near the wall, was also listening intently to the hobbit-talk. He had a tall tankard in front of him, and was smoking a long-stemmed pipe curiously carved. His legs were stretched out before him, showing high boots of supple leather that fitted him well, but had seen much wear and were now caked with mud. A travel-stained cloak of heavy dark-green cloth was drawn close about him, and in spite of the heat of the room he wore a hood that overshadowed his face; but the gleam of his eyes could be seen as he watched the hobbits."