How is the cargo hold volume for passenger aircraft designed?
The cargo compartments are not only for luggage. Commercial cargo also travel on passenger aircraft, besides cargo aircraft. With luggage alone, there are plenty of spaces in the cargo deck.

In fact, if you fill all the seats, all the cargo space and sufficient fuel for a long trip in a B777, the plane won't takeoff - its GW (Gross Weight) exceeds the MTOW (Max Take-Off Weight).

Airlines are smart people - if a flight is not full, dispatch will put more cargo on the aircraft to utilize it. A lot of passenger airliners also offer air delivery service.

Some passengers complain, "I can't book the flight. The staff says it's full, yet there are many empty seats!" Commercial cargo yields more revenue than an economy seat. Thus, given the weight limitation, an airline may prefer cargo vs passenger.

I don't have info about A380, but I'd guess that the A380 has less extra cargo space than the B777. But as long as it's enough, it isn't a problem.