A cumulonimbus cloud's precipitation rate is typically heavier due to it's updraft being much more robust, which in turn means a heavier condensation rate. As all these water droplets pile up quickly, they become too heavy and fall at great rates. Nimbostratus clouds are a lot of the times formed by isentropic upglide, and this slow gradual lifting process results in a much slower and steadier condensation rate. This means more gradual lighter precipitation.