The 500 tonnes is not a compact object sitting in the sky. Where the 500-tonne estimate comes fromThe calculation starts with two estimates: the cloud’s volume and its liquid water content. Whether the parcel keeps rising depends on the balance among temperature, humidity, liquid water, pressure and mixing. Change the temperature or mix in drier air and that boundary moves, even if the surrounding air mass continues on. So the apparent contradiction is real only if we imagine 500 tonnes gathered into one place.