hammerb32,
You're correct, essentially with the A380 you have the cargo capacity of a single deck widebodied aircraft. The increase in passenger numbers (and therefore luggage) to the cargo capacity is disproportionate so you do indeed lose a fair chunk of cargo capacity.
B77W cargo capacity volume is 216m^3
A380 cargo capacity volume is 184m^3
...shows it's quite a significant amount too when you take into account that you have a smaller volume with a larger number of passengers.