[saosebastiao]

It's simple diminishing returns to size efficiency.

There's a saying in aviation that goes something like "it takes a lot of fuel to fly a lot of fuel". Even over the course of a single flight, the fuel consumption rate goes down significantly as that fuel gets consumed. You can fit more stuff in a bigger plane, but it takes more fuel to fly the 100,000th pound than it does to fly the 100th pound, so you only want the larger capacities if you know that you are going to consistently utilize them.

[Reason077]

That's certainly a factor, but there's more to it.

- A380 is heavier, per seat, than newer aircraft designs which are making use of advanced composites, new technologies that reduce the weight of wiring, etc.

- A380's engines are older models which are less efficient than the latest-generation engines fitted to the 777X and A350

- Aircraft with 4 engines have higher operating costs than 2. Additional fuel burn, maintenance expenses, etc.

Obviously some of this could be improved if Airbus were to have developed an upgraded version of the A380 (and Rolls-Royce it's engines). There was talk of both a new-engine A380neo and a stretched A380-900 at one point. But clearly, the market was judged to be too small to justify the substantial development costs.

[ThenAsNow]

While the points you make are true, they don't address the parent's hypothesis about engines, which is true. Once you get into commercial jetliner engine sizes, the fewer the number of engines for a given thrust level, assuming equivalent internal technology, the lower your fuel burn will be. It's a matter of minimizing unavoidable per-engine losses, like bearing losses, cooling losses, endgap leaks in rotor tip regions, etc. These kinds of losses do amortize well with engine size.