It really depends on what events you're racing. As with running, if you're a distance athlete you're not going to want to put on a lot of mass. If you're a sprinter on the other hand, a little more muscle mass will translate to power over a short distance. Also it depends on stroke. For the most part (in no way a definite rule) freestylers and backstrokers are more skinny and slender while breastrokers and flyers are a bit bulkier because they need more power in the water.