The airframe of the A330 features a low-wing cantilever monoplane with a wing virtually identical to that of the A340-200/300. On the A330-300, one engine is installed at the inboard pylon while the outboard pylon position is not used; for the A340-300, both engine pylons are used, which allows the A340-300 wing to sustain a higher (wing-limited) MTOW. This is as the A340's two engines at each wing provide a more equal force distribution (engine weight) over the wing, while also the total engine weight counteracting moment is located more outboard with more engine weight located further outboard on the wing, hence the wing root bending moment with equal TOW is less on the A340-300 than on the A330-300. The A340 has a longer range and heavier payload, while the A330 has better fuel economy over the same distance.
The airframe of the A330 features a low-wing cantilever monoplane with a wing virtually identical to that of the A340-200/300. On the A330-300 one engine is installed at the inboard pylon while the outboard pylon position is not used; for the A340-300, both engine pylons are used, which allows the A340-300 wing to sustain a higher (wing-limited) MTOW. This is as the A340's two engines at each wing provide a more equal force distribution (engine weight) over the wing, while also the total engine weight counteracting moment is located more outboard with more engine weight located further outboard on the wing, hence the wing root bending moment with, equal TOW is less on the A,340-300 than on the A330-300. The A340 has a longer range and heavier payload, while the A330 has better fuel economy over the same distance.