Steve Munatones explains what happens to the human body post a 10K race. He poses the most logical reason to date as to why Grant Hackett appeared so plushy. From Lane 9 news: "... All of the causes of Hackett's bloating are common among open water swimmers and are more notable and noticeable based on the water salinity and duration of exposure.
"... As often happens in salt or brackish water, the body naturally swells from the prolonged exposure. This is why many English Channel and marathon swimmers look bloated and "soft" when they exit the water after a long swim. Many swimmers appear out of shape, especially around the stomach, hips and thighs, after long open water swims. ..." [Link]
He includes other reasons and discusses a concept called the "third spacing" effect. Here is a Wikipedia entry on "third spacing." [Link]
The situation is still saturating the Australian press. Before and after photo and article from the Sydney Morning Herald which blames "carbs" rather than the more likely explanation by Munatones: [Link]