It's anime norms that boobs can undergo compression and be made smaller the more they're covered by clothing, to the point they can even be made even nonexistent for disguises no matter their original size.
Same thing happened to neon and pepper in their summer outfits.
Right. Pepper freaking blew my mind. Neon, is understandable she didn't get too big but there's no way that loose sweater of her's hid Peppers...."firepower"