A "cc" is a measure of volume
You have raised some very ineresting issues in your question.
First of all as mentioned in the title, breast implants should be considered in terms of their volume and not their weight. To clarify this, you can imagine that a pound of feathers would occupy a lot more space than a pound of metal.
Implants are generally considered in terms of their volume. A "cc" is equal to a Milliliter or "ml".
Based on your question, I would have to assume that you had a 280...