Small pipe

Small pipes  are tiny Warp Pipes that require the player to be tiny in order to enter. Half the size of a regular Warp Pipe, they made their debut in Super Mario Bros. 3.

Super Mario Bros. 3
Small pipes first appear in Super Mario Bros. 3, in World 2-1. They appear with a purple coloration and do not serve any purpose other than being decoration.

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. sub-series
Small pipes appear in every game of the New Super Mario Bros. sub-series starting with New Super Mario Bros. itself, where they make their first functional appearance. They act like normal pipes, but can only be used by players in their Mini form. Star Coins are often hidden inside rooms that can only be accessed via these pipes.

Super Mario Maker 2
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
A single small pipe appears along Bowser Path in Mario & Luigi: Bowser's Inside Story and its remake. Mario must use the Mini-Mario technique to enter it and reach the ? Block containing the POW Mush Jam.

Super Smash Bros. series
Nonfunctional small pipes appear on the Yoshi's Island stage in Super Smash Bros. Melee, Super Smash Bros. Brawl, Super Smash Bros. for Wii U, and Super Smash Bros. Ultimate, supporting a larger diagonal Warp Pipe. Their appearance is based on Super Mario World.

Trivia

 * In New Super Mario Bros. Wii and all subsequent New Super Mario Bros. games, when a mini character enters a small pipe, they exit out of a normal-sized one. This is likely due to the fact that other players not in the correct form can exit the same pipe in multiplayer.