It's called a sports bra. Serena Williams is a very ample-chested, curvaceous woman, but she's also one of the most powerful, fast, and agile athletes in tennis (and not just women's tennis). So no, it's not pure fantasy. Ample breasts can be an encumbrance, but that encumbrance can be overcome with sufficient athleticism and the right athletic wear. (I mean, think about it. There's a free-swinging portion of the male anatomy that can be quite awkward if it's not properly contained and protected, but that hasn't kept men from succeeding athletically, because they devised garments and equipment that could compensate for the problem.) Although, granted, it's hard to see how Diana's bustier could work effectively as a sports bra.