呢度答咗
https://www.flexport.com/blog/airbus-a380-no-cargo-equivalent/
"An A380-F would be too fat to fly at a profit: The plane would hit the maximum payload (a constraint of weight) before its maximum cubic space (a constraint of volume). Its design can’t support the maximum payload required to generate a profit.
Consider a comparison with the Boeing 747-400F, a popular air freighter. The 747 has a maximum take-off weight of 448,000 kilograms to the A380’s 575,000. In addition, the 747 has a cargo capacity of 710 m3 to the hypothetical A380-F’s 1134 m3.
The A380-F would be able to carry 60% more volume than the 747, but only 28% more weight. It wouldn’t be fully loaded at typical levels of air cargo density, or at least nothing close to what can be supported by the thrust capacity of the 747."