Why do rain droplets differ so "much" in size? Why do rain droplets differ so "much" in size? From the ones in drizzle rain to the droplets in a heavy shower?
I guess there is a complicated interplay between the height from which they fall, the humidity, the wind speed, friction and things I've overlooked.
If the air is very humid (coming straight from a warm ocean) after which the air enters land, above which the air is much colder (which can contain less water vapour), I can imagine that big drops condensate (of course they start out small but increase in size fast due to condensation), because of the big difference in humidity and temperature. When the fall down (for a great part by condensation), they can even get bigger by merging with other drops, or breaking up if they become too big. The wind plays obviously a role here. It lets drops collide with each other, which makes them smaller. But this effect is less if the drops are heavier.
On the other side of the size spectrum, we have drizzle rain. These drops stay little, meaning they haven't merged too much with other drops. It's like their distances to each other don't grow that much. I guess they don't originate at a great height, and the wind keeps their mean distance to each other more or less the same (in contrast with the big drops, which have a relatively greater difference in size).
Middle-class drops, which fall mostly during long periods of rainfall (except maybe in the monsoon rains), are an in-between case.
These are my own thoughts, but I'm sure there are people who know more about this.
 A: According to a French researcher, raindrops differ in size because of topological changes they undergo as they fall.  They start out spherical when they first form within clouds.  Surface tension maintains that shape.  However, as they fall and encounter air resistance they flatten like a pancake, and then air pressure pushes up the center and they become umbrella-like.  As air pressure builds inside the inverted cup shape of the falling raindrop, eventually it explodes into a chaotic shower of smaller drops of various sizes.
Mist rising from near ground level is very fine and uniform compared to raindrops, as mist droplets do not fall from a great enough height to undergo topological change.  Similarly, the super-large raindrops that fall in the still air that precedes some thunderstorms, form from warm moist air drawn from ground level into low-lying clouds prior to the violent downpour of chaotic droplet sizes that follows the advancing weather front.
A: The size of a rain drop depends on the process of formation. In principle, condensation of moist air at high altitudes causes tiny drops to form. As soon as those drops get big enough, they start to fall. Now if these drops encounter other little drops on their way down (big drops fall faster, so they catch up with little drops), they will "eat" the little drops and get even bigger. They will keep doing this until they reach the ground.
At the same time, when a drop gets sufficiently large, it will deform and can "explode" back into a bunch of smaller drops. An interesting article including a slow-motion video of this process can be seen here
Turbulence in the air plays an important role in this formation process. When the air "changes direction" a lot, smaller droplets tend to be dragged along with it - while larger drops have greater inertia compared to their drag. This means that under turbulent conditions, smaller drops will slam into bigger drops (where under calm conditions the big drops go faster and "eat" the little drops).
The result is that for a given moisture content in the air, and altitude / thickness of the clouds, you will get a certain size of rain drops. The thicker/denser the clouds, the more drops from the top can grow and absorb moisture in lower layers. And when there is more wind, it also increases the drop size as explained by the turbulence argument.
A lecture exploring this in more detail can be found here - see especially slide 7 and further.
