Cumulus clouds, which are generally the largest, can weigh in excess of 1 million pounds. This figure comes from a relatively simple density calculation involving the average volume of the cloud times the known density of water within it.
A large cumulus cloud can easily reach a size of 1 kilometer to a side, or 1 cubic kilometer in total. A cubic kilometer is equal to 1 billion cubic meters, and each cubic meter is likely to hold roughly 0.5 gram of water in the form of tiny droplets. Water, which weighs around 1 kilogram per liter, doesn't immediately fall out of the sky because the droplets are small enough to be supported against gravity by the surrounding air, which is often warm and full of updrafts. Taking a density of 0.5 gram per cubic meter and multiplying this by 1 billion cubic meters, a cumulus cloud holds around 500,000,000 grams of water. Half a billion grams is equal to 1.1 million pounds, or roughly the weight of 100 elephants. When the convection driving the formation of the cloud shifts or shuts down, or when the dense, relatively dry air outside of the cloud stops supporting it, the water inside the cloud condenses and falls as rain.