there are some elite level cyclist women that are pretty big too. its one of my great 'endurance sports mysteries' - do they NEED the extra mass to get the power, or do they HAVE the extra mass because its a niche sport where you can get away with it?
but yeah, won't work for running.
They're big but not THAT big. She's carrying wayyy more extra fat (not muscle) than any women's pro cyclist for sure. The cyclists do have some big quads, particularly track sprinters, but they certainly have little to no body fat. That swimmer is at least 18% BF, if not more, which is downright pudgy for a typical cyclist or runner.
I'm sure the 'fat' helps in swimming, very analogous to a wetsuit. Fat floats, and you expend less energy on flotation. Even if it saves you 0.25% of energy, in a long race, that may be a crucial advantage to winning. As well, the extra fat may be a lot of 'brown' fat which at least theoretically, could help you generate body heat and possibly work at a more efficient temperature for those easily chilled extremities/etc, but that's just a theory.
Maybe not as much for a sprinter or pool swimmer like Phelps, but I think that marathon swim champion physique pretty much shows this is the case for long-distance swimming at least.