Whenever i see a pic or vid of a muscle girl crossing their arms it makes them look huge and i love it. especially when they have big pecs and forearms. I wanna see more of those!
I like when the girls have their arms crossed for a different reason: it makes them look more "confident". Combine this with wearing a one piece swimsuit (you can tell I have a thing for girls wearing one piece swimsuits), and the girl fits the superhero image.
Also, girls with both hands on their hips enhances their look of confidence as well.