I know a guy as well who does ballet, studies it in college right now. For one, there's a lift you probably know very well; happens a lot in couples figure skating too. Effortlessly lifting someone who weighs about 100-120lbs, without making an I'm about to shit myself face, and easily enough to lift that person over your head?
Ballet dancers land like feathers, practice hours a day heavy stretching with some heavy jumping around, they hold positions for long periods of time that would be otherwise incredibly difficult to do so - and by which I mean minutes on end. And they not only practice that stretching and holds for long periods of time but they use that to practice their dancing in a slow manner as well. It's all in how you define gainz I reckon.