The answer is that what you typically think of as your “weight” is actually the force of gravity exerted on you by the Earth minus the buoyant force exerted on you by the air. If you were weighed in a vacuum, you would weigh something like 0.1 kg less, because air exerts a force on you proportional to the amount of air you displace. Similarly, if we weighed Wailord in a vacuum, we would see that it exerted a force of about 3900 Newtons, equivalent to the force of gravity on an object with a mass of 398 kg. However, if you tried to weigh it not in a vacuum, it would just start floating upwards, and indeed it would apply a net upwards force, due to the buoyant force from the air being greater than the force of gravity.