Fatso is presumably the youngest of the brothers (although this is never clarified in the film) and he loves to cause mischief or sing. He also loves to eat, as is pretty obvious. He's loyal to Stretch but seems a little more independent than Stinkie.
Fatso is voiced by Brad Garrett, who also voiced Dim in A Bug's Life Bloat in Finding Nemo. Gusteau in Ratatouille, Riff Rafe in Underdog, Luca in Garfield: The Movie. The Giant in Hoodwinked Too! Hood Vs Evil. Plumber in Stuart Little 2.