Banelings said: Yukarin without a chest? UNACCEPTABLE
Actually, there's a logical explanation behind it. The goal was to relax in the bath, right? Well, larger breasts cause more strain on a woman's back, so for the purposes of relaxation, she manipulated boundaries so as to have smaller breasts for the sake of relaxing.
Through the use of her boundary hax powers, her breasts can be as large or small as she needs them to be. She's not here to be sexy, but out in public expect those things to grow.