Sumo wrestlers are popular athletes in Japan. They are known for being overweight men who only wear a silk loincloth called a mawashi. The rest of their body is completely bare. Their hairstyle is made into a topknot style known as a chonmage.
If you are just a teenager or child, there are sumo wrestler costumes that you can purchase from novelty stores. These costumes already come with a complete body suit that is stuffed with cotton to make it look big. However, this will take away from the authenticity of the look. It is better to purchase a more deluxe fat suit from Amazon or at a professional Halloween store that sells quality-made costumes.
If you want to make your own fat suit and don't want to spend hundreds of dollars on a professionally made one, purchase an extra large long-sleeve shirt and pants which have the same color as your skin. Use a permanent marker to draw on the pectoral muscles of the torso and quadriceps of the legs. Now put on these clothes and stuff them on the inside with bags of cotton. This should create the fat appearance that you are looking for.
For all overweight men out there, you will not need a fat suit. You can finally use your excess weight to create an authentic looking sumo wrestler costume. All you need to do is obtain a silk loincloth and wear it around your waist and in between your buttocks. If you have long hair, then get it styled into a topknot. Otherwise, wear a topknot wig if you can find one.