I used to swim through highschool and it's not something that they are trying to do as a strategy, it's just how your body ends up. Most swimmers even on the high school level eat MASSIVE meals. Imagine a 120 lb girl eating 4000 calories or more a day, my team had a few girls like that. They don't gain any weight, they needed that to maintain body mass. Your body just wants food all the time so you just keep eating. The practices aren't designed to be like marathons because that will cause your body to eat muscle (unless you do extreme distance). You want to balance the cardio with things to build and train the muscles specific to swimming. This will have you keep a bit more bodyfat than a runner or something similar, but they will be better conditioned for their events.
Just remembered one more bit that would affect why they don't look as ripped. The muscles they target in their training aren't as noticeable. Core body strength is key, you don't need massive biceps to swim. This will make them look less muscled because their arms just won't be that big. The biggest muscle that's visible is the lat, it connects from the back of your shoulder to your back and side (grab the back of your armpit, that hunk of flesh is your lat). In swimmers it will grow much larger than other sports that don't require the same motion.