NWSRG:
A simplified answer is the A380 has a big wing which can create a lot of lift, so, it doesn't need a lot of thrust. Also, Airbus planes tend to have better low-speed performance than an equivalent Boeing airplane. Boeing planes tend to have better high-speed performance. That's why Boeing planes can cruise faster than an equivalent Airbus plane, yet they don't burn more fuel.
The A340-300 has a relatively small wing (highly loaded) and relatively low thrust. (You can compare the wing loadings, MTOW/wing area, of different aircraft, then you'll know what I mean.) That's why it has been the center of a lot of slow-climb jokes. Nevertheless, it was adequately designed for its intent.