There are no specific physical requirements for sumo. The rules and culture of sumo reward being as big and strong as possible. There is no benefit to being lean or cutting body fat, so rikishi (sumo wrestlers) simply don't. They get brutally strong, and have simply enormous muscles hidden under their fat. Many pro sumo wrestlers match or exceed the muscle mass of pro bodybuilders; you just can't see it because they don't go through the extreme and frankly insane fat- and water-cutting processes that bodybuilders do to prepare for a show.
What happens in sumo is what happens in other strength and power sports: in lower weight classes, athletes are lean in order to be as strong (and big) as possible while remaining within the upper bound of their weight class. Surplus fat is extremely rare. For instance, Chinese olympic weightlifters Lu Xiaojun (77kg), Tian Tao (85kg), Liao Hui (69kg) all stay lean while getting very strong:
But at the highest weight class, or in sports like American football where there are no weight classes, the drive to be as strong as possible is not restricted by mass, and so athletes are quite large and have plenty of excess fat. (This proves how this is not an east/west phenomenon; NFL linebackers are not known for being lean, but athletes in running positions can be quite lean.) This is simply because there is no benefit to losing the fat, and plenty of drawbacks: it would take time and training effort, and would interfere (even if only slightly) with maximum muscle gain. So superheavyweight fighters and Olympic lifters are often chubby, even fat, such as the incredibly strong Olympic lifter Chingiz Mogushkov, seen here not having time to worry about visible abs because he's about to lift 250+ kilograms over his head:
Or heavyweight MMA fighter Roy Nelson, who likes to eat and punch people really hard:
However, the fact that sumo rewards being as strong as possible--and even as big as possible, because it makes the athlete hard to push or move--does not mean that sumo wrestlers are uniformly fat. Some of the best rikishi of all time have been incredibly strong but also agile and relatively lean. My favorite is Chiyonofuji, who reached the rareified top rank of yokozuna:
If you think this wrestler is not muscular, please remember that he was 127kg and 183cm. That is a very, very muscular athlete; he is simply at a healthy and strong body fat percentage. (For those not used to recognizing muscular builds outside of bodybuilding, it helps to look for trapezius muscles between the shoulders and neck, or at the back and glutes.) Chiyonofuji was an incredible champion who looks even more jacked on video in action than he comes across in pictures.