When a Poké Ball hits the Pokémon, as long as it is not deflected, the Poké Ball will open, convert the Pokémon to a form of energy, pull it into its center, and close.
The larger size makes throwing the ball easier, while the smaller one makes for easier storage on a belt clip, in pockets, and in Bags.
As mentioned, the generic Poké Ball design is not constant and has been remodeled and altered innumerable times in order to create new Poké Balls that are adapted for specific conditions.
A Poké Ball (Japanese: モンスターボール Monster Ball) is a type of item that is critical to a Trainer's quest, used for catching and storing Pokémon.
Both a general term used to describe the various kinds as well as a specific term to refer to the most basic among these variations, Poké Balls are ubiquitous in the modern Pokémon world.
It also claims that the Pokémon Primeape and its notoriously violent nature was directly responsible for their creation.