The A380's maximum takeoff weight is 575,000kg, compared to 447,700kg for a 747-8 freighter. However, the A380 -- sitting empty on the ground -- weighs 80,000kg more to begin with, and also requires more fuel (because of its greater mass).
The result is that the useful payload of an A380 is lower than a 747-8F (about 84,000kg versus 132,000kg).
And that's without getting into the other issues, like the A380's structural middle-deck (which can't be removed to turn the interior into a giant cargo hold) or inability to have its nose turned into a large cargo door like a 747 (the A380's flight deck is too low to the ground, in order to maintain training compatibility with the A330 and A340).
The result is that the useful payload of an A380 is lower than a 747-8F (about 84,000kg versus 132,000kg).
And that's without getting into the other issues, like the A380's structural middle-deck (which can't be removed to turn the interior into a giant cargo hold) or inability to have its nose turned into a large cargo door like a 747 (the A380's flight deck is too low to the ground, in order to maintain training compatibility with the A330 and A340).