Yuyuko's kimono must be imbued with some breast-enlarging magic. (Maybe she's secretly a flattie.) Also, those are definitely larger than medium.
That's just Nori Tamago's normal way of drawing Youmu. They usually draw her with a rather nice figure, but not as endowed as they tend to make Yuyuko herself. That said, Youmu looks good with a nice figure and I always knew she would look good when she grew up, so they work quite well on her.