It is about buoyancy - which is about the weight of the fluid volume displaced, not about the weight of the displacing object. In the case of lead in water, the bouyant force is equal to the weight of the water volume displaced - which is far less than the weight of the lead, and the lead sinks. And mountains do sink into the mantle until the buoyant force (equal to the weight of the displaced mantle volume) equals the weight of the mountain.