Fat is Madotsuki's down special move.
Madotsuki activates the Fat effect, increasing her body mass, weight, and falling speed. When she jumps and lands, she creates a small earthquake, dealing 17% damage from contact and 8% to anyone standing near her. Pressing A will make her stomach growl, dealing slight damage to nearby enemies; this can also be done while walking and jumping. Her normal side smash is replaced with a headbutt during this mode, having lots of range and power at the cost of slight lag. Madotsuki will slim down after pressing B or waiting 15 seconds, and she can become fat again after 10 seconds have passed.
Fat is one of the effects in Yume Nikki, obtained by interacting with Strober at The Docks. When in this effect, pressing the 1 key will make Madotsuki's stomach growl, accompanied with an emote. Aside for completing the game, it has no practical use.