No, rain is water that has condensed in the atmosphere by way of the water particles bumping into each other and making the drops (mass) bigger. When it is big enough to be effected by gravity, it falls to earth, on the way down it may or may not bump into more water particles effecting the mass. How big a drop is depends on how many collisions it has on the way down.
It can also pick up impurities out of the air on the way down, ever notice how clean the air smells right after a rain?