since when did starchy carbs become evil?
only the ability to not overeat them is the problem. they are very nutrient dense...and your body runs on long chain polysaccharides! ive never heard so many reasons to disregard the laws of thermodynamics. calories in vs calories out. gymnasts dont bulk up/put on any more LBM than usual, they are very lean to begin with and maintain a decent amount of muscle mass. the leaner you are, the bigger/harder you look. they arent gaining by leaps and bounds due to some miracle of macronutrient manipulation. if there is some muscle gain it is slow inseason or done in the offseason (where they can get "fluffy").
this is not to say there wont be elite athletes that follow diets the mimic precontest bodybuilders, but as for them growing and losing at the same time....its no different than any other person on a calorie deficit, it doesnt happen much or very slowly.