Small pipes first appear in World 2-1 in Super Mario Bros. 3. They appear with a purple coloration and do not serve any purpose other than being decorative structures. Small pipes also appear in World 7, where they are green and may have Munchers pop out of them. Horizontal cannons and a type of forward-facing Rocket Engine resemble dark small pipes.
Super Mario World
Small pipes reappear in Super Mario World, serving the same purpose as in Super Mario Bros. 3. They are occasionally seen supporting Pipe Cannons.
New Super Mario Bros. series
Small pipes in New Super Mario Bros. and its sequels often lead to rooms with a hidden Star Coin and usually also shortcut rooms with coins that can be used to skip some parts of the level. As of New Super Mario Bros. Wii, exiting from a small pipe leads players out of a normal-sized one, likely due to the fact that other players not in the correct form can exit the same pipe in multiplayer.
Small pipes reappear in Super Mario Maker 2 in the Super Mario Bros. 3 game style as the ground tiles in the forest and desert themes, being green and purple respectively.
Super Paper Mario
Small pipes appear in Super Paper Mario. The player has to use Dottie to shrink their character so they can enter them, similar to the use of Mini Mushrooms in the New Super Mario Bros. games.
Mario & Luigi: Bowser's Inside Story / Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey