Explanation:

Clouds are made of tiny water droplets and ice crystals up in the atmosphere, when this droplets become too heavy that the cloud (or the room) can no longer hold it you, then it starts falling. As it falls it eats up even more droplets.
We can call the growing droplet a raindrop as soon as it reaches the size of 0.5mm in diameter or bigger.
If it gets any larger than 4 millimeters, however, it will usually split into two separate drops.The raindrop will continue falling until it reaches the ground. As it falls, sometimes a gust of wind (updraft) will force the drop back up into the cloud where it continues eating other droplets and getting bigger.
When the drops finally reach the ground, the biggest drops will be the ones that bumped into and coalesced with the most droplets. The smaller drops are the ones that didn't run into as many droplets. Raindrops are different sizes for two primary reasons.
1. initial differences in particle (condensation nuclei) size
2. different rates of coalescence.