When a guy's fat gut is so huge it resembles a hippopotamus head ... it therefore must be a hippopotamus that only has 2 legs and has purchased itself a lovely man's head for a hat (probably not such a lovely hat given the size of the gut)
An unnecessary addition to something that was previously working without it--like putting a hat on top of another hat. In comedy, this is used when placing two funny things too close to each other so that they distract from each other.