Cassandra doesn't have the brawn, raw strength or toughness her sister Gia does, but her overall power level is considered to be the higher of the two. Cassandra has just enough strength to maneuver her body no matter how big or heavy she wills her breasts to become but once she reaches titanic levels significantly larger than this, she is basically immobile, unlike her mother, Mara who can move at any size, including her current room filling role in the basement of Cleavage LLP. Cassandra can conceivably grow larger than her mother and she can do so instantaneously like two massive air bags going off causing devastating catastrophic damage to anything within her blast area. Once she reaches that size, she can't push or move or destroy anything heavier or denser than her with her body or breasts, but the initial shockwave of her rapid increase in size always does the job which is why she is reluctant to release her energy or grow in size comparable to Mara.
I'm not quite happy with the perspective of the left boob and may work on this some more, but I feel like this is a good step to getting better a "wow" size characters like Cassandra and non-Gaia Marlin and Diane.