The A380-800 was vastly overbuilt, as it is, in essence, a shrink. Shrinks are always the least efficient airplanes in a family. Usually larger variants are mor

The A380-800 was vastly overbuilt, as it is, in essence, a shrink. Shrinks are always the least efficient airplanes in a family. Usually larger variants are more efficient, while smaller variants tend to be more capable (in terms of range, take off performance, etc.). 787-9/10 is a prime example of this, as the 787-10 is just a longer 787-9: the 787-10 can seat more and is quite a bit more efficient than the 787-9, but the 787-9 can fly a lot farther. The A380 was optimized around a longer variant, the A380-900, with enough margins for an even longer version (an hypothetical A380-1000). Turns out even the A380-800 was too big, so the A380-900 was never built. But the A380-800 still had to carry most of the structure of the heavier variant, making it grossly over-built. But that doesn't fully explain why the A380 is so bad for cargo compared to the 747-8F: that's due to the "floor" that is an integral part of the structure of the A380. An 747-8F is just an empty tube.