Why do hailstones come in varying sizes?

Hail is formed when raindrops pass through a belt of cold air and are frozen during their passage to earth. Single raindrops form small hailstones. Sometimes as the small hailstone falls it meets a rising current of air. Then it goes up to the level where raindrops are formed. New drops cling to it and as they fall through the cold belt they form an enveloping layer and freeze, enlarging the stone.