From the Super Mario Wiki
Super Sushi is a powerful food product created by Dr. T. Garden; whoever eats Super Sushi grows twice as big and powerful. How Super Sushi is made is unrevealed, though the use of a microwave-like device is shown to be needed.
In The Super Mario Bros. Super Show! episode "Mario Meets Koop-zilla", King Koopa, after Dr. T. Garden creates his most powerful batch of Super Sushi, steals and devours it, causing himself to grow gigantic in size. Super Sushi later appeared in the episode once more, when Mario, believing eating Super Sushi to be the only way to defeat Koopzilla, has Dr. T. Garden make him a batch of it, which Mario eats with spaghetti.
Dr. T. Garden later created a cure for the effects of Super Sushi, the liquid Super Sushi Shrinker, which must be administered via injection to someone who has eaten Super Sushi in order to reverse the enlarging effects of it.