Why does the cathode gain more than the anode [loses]?

A. Hi, Kurt.

Your title says the opposite of your text, so we don't know which is the case. You will see, of course, that no copper is created, and whatever difference there is between the amount gained on the cathode versus lost from the anode will manifest as a change in concentration of the solution.

Although it's probably more common for the anode efficiency (the percentage of applied electricity that results in metal deposition/dissolution rather than generation of hydrogen, oxygen, or OH) to be higher than the cathode efficiency, it can go the other way too (for example, in nickel plating the anodes can passivate and stop dissolving).

It is truly complicated because both efficiencies will depend upon the copper concentration and the pH, whereas the pH will also start varying as the solution picks up H or OH due to the inefficiency unless you add acid or alkali.

If you ran long enough, I think you would arrive at equilibrium, but don't feel bad if this is impractical; industrial platers who have been running for years often do not allow equilibirium because better plating may result at a non-equilibium concentration.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey