Why is sumo used for?

Answer 1

Not all of them are fat.

Many are large so they can use their weight to push the other person out of the ring.

They'd never stop fattening themselves.

Answer 2

It increases their mass, which makes it more difficult to move them. The object is to move them out of the ring, or to make them fall, the increased mass makes this harder to do.

Answer 3

It deals with mass and the ability to be difficult to move. They can use their weight to advantage to push their opponent out of the ring.