Mesosoma had mad a very big miscalculation. The Changeling Affection Ceremony wasn’t designed with Royal changelings in mind, regular changelings when they store love have an upper size that they reach. While there is some variance between individuals the maximum size is usually similar, royals though don’t have an upper limit and can theoretically grow indefinitely.

So when Mesosoma finally dropped her changeling glamour restricting her breast size she was in for the shock of her life when she just continued to grow. The positive thing to come out of this though was it showed how much love there truly was in this relationship.