At times raindrops are bigger and sometimes smaller

Rain happens when warm, moist air rapidly cools at high altitudes in the presence of surrounding colder air. At any given temperature, dry air can accommodate only a certain amount of water vapour.

As the moist air temperature drops, due to cooling, it becomes super-saturated with water vapour. When air can accommodate no more water vapour under equilibrium conditions, it is saturated.

Under super-saturated conditions, air holds more moisture than it possibly can under equilibrium conditions. Hence, there is the tendency of air to precipitate out any excess moisture present and it rains.

Rain drops form due to condensation of water vapour around preferential sites such as microscopic dust particles, pollen grains etc. that are present in air.

As the drop-wise condensation proceeds, microscopic droplets of water initially formed grow in size and the water droplet begins to fall under the action of gravity.

While falling, the droplets can collide with other droplets and coalesce to form bigger droplets.

Alternatively, droplets can collide with other droplets and break apart into smaller droplets due to the impact. There is another reason for the break-up of water drops.

Due to the relative velocity between the falling water drop and the air around it, viscous shearing forces develop on the surface of the drop.

These drag forces cause the drop to distort in shape. During its fall the waterdrop accelerates, and the magnitude of the drag forces increases roughly in proportion to the square of the fall speed of the drop.

Due to shape distortions and the rapidly increasing effects of the shear stresses on the air-water interface of the drop, the drop may break apart.

Large drops of water have a tendency to break apart more than the smaller ones. Due to these reasons growth of drop size due to coalescence, and break up due to collisions and viscous shear rain drops come in varying sizes, from the smallest to the largest, by the time they hit the ground.