Small Mario

Small Mario (formerly referred to as Regular Mario or just simply Mario ) is the weakest form of the player character in the main Super Mario games. This form does not have any special abilities, and it also does not take any power-ups to assume this form, unlike other forms. Super Mario turns into Small Mario when he is hit by an enemy or obstacle. The player almost always starts the game off in this form (Super Mario Bros. 2, Super Mario 3D Land, Super Mario 3D World, and Captain Toad: Treasure Tracker are notable exceptions).

Any damage taken in Small form will cause Mario to lose a life. Usually, before being able to assume even stronger forms, Small Mario first must become Super Mario by eating a Super Mushroom. Being Small Mario can sometimes be useful; in some games, such as Super Mario Land, there are small crevices that can be passed through easily with Small Mario.

Super Mario series
In Super Mario Bros., Super Mario Bros.: The Lost Levels, and the original and Super Mario All-Stars versions of Super Mario Bros. 3, if Small Mario gets a higher-tiered power-up (like a Fire Flower), the power-up turns him only into Super Mario, but as of Super Mario World, this was changed so that, for example, grabbing a Fire Flower turns Small Mario right to Fire Mario. Also, in Super Mario Bros., Super Mario Bros.: The Lost Levels, and the original version of Super Mario World, taking damage as Fire Mario (or Cape Mario in the latter game) immediately shrinks Mario down to his Small form.

In Super Mario Bros.: The Lost Levels, Super or Fire Mario can also become Small Mario by touching a Poison Mushroom. This form also appears in most other 2D Super Mario games with the same effects.

In Super Mario Bros. 2, Mario, Luigi, Toad and Princess Toadstool start out each life in Super form (at the start of each level, they only have two hit points; they start with the current maximum hit points full after defeat), regardless of what their form was upon defeat or exiting the previous level. They become small if they only have one hit point left. By finding the two Mushrooms hidden in Subspace, the characters can extend their maximum hit points to four; also, upon the Mushroom's collection, the character always regains full health.

In Super Mario Advance, however, the Small form persists between levels (unless the player finished the previous level in their Super form, in which case it transfers to that level for that life only, even if the player switches characters), the characters start the game and each life in their Small forms with only one hit point remaining (of the current maximum), rather than their Super forms, and the maximum hit points per level is five, due to an added Mushroom per level. In the original NES version, when characters are in their Small forms, only their lower halves shrink while their upper halves remain intact. In the All-Stars remaster and Super Mario Advance, this was changed so that their entire bodies shrink. As a result, Peach is also affected, as her hair becomes shorter as she shrinks into her Small form, rather than remaining intact like in the original NES version.

In New Super Mario Bros. Wii, the Toads to be saved in the levels become Small Toads after being hit once by enemies.

Bowser is turned small in the ending of Super Mario Galaxy 2, possibly as a side effect of using the power of the Grand Star's to make himself giant. When in this form, he could be jumped on by Mario and Yoshi, and could be hit by Yoshi's tongue. In Super Mario 3D Land, Small Mario makes its 3D Super Mario debut, as previous 3D Mario games had a health meter. In this form, Mario and Luigi lose their caps, making this form similar to Wario's Small form, as seen in Wario Land: Super Mario Land 3 (unless the player collects the maximum amount of lives (1,110 lives), which has Super Mario/Luigi lose the cap instead). Their voices are also higher-pitched. However, like in Super Mario Bros. 2, Mario and Luigi start out each life in their Super forms. Additionally, in Super Mario 3D World, Peach, Toad, Rosalina, and Captain Toad can also get affected by this transformation, which works as it does in Super Mario 3D Land. As such, Peach and Rosalina's hair gets shorter (like in the remakes of Super Mario Bros. 2), and the Toads' cap colors are reversed.

Captain Toad: Treasure Tracker
Captain Toad and Toadette's Small forms appear in Captain Toad: Treasure Tracker, retaining the features from Super Mario 3D World (including the reversal of cap colors and higher-pitched voices). However, as in the original and All-Stars versions of Super Mario Bros. 2, the Small form does not persist between levels; every level is started in Super form, regardless of what form the previous level was finished in.

Mario Golf series
When Mario gets a Double Bogey or below in Mario Golf: World Tour as well in Mario Sports Superstars, his animation depicts him getting hit by a shell, which turns him into Small Mario.

Super Mario Run
In versions of Super Mario Run prior to the Fall 2017 update, Peach, Toad, Toadette, and the Yoshis are defeated in one hit and therefore do not have Small forms to shrink into when getting hit. In versions since the Fall 2017 update, they are now given Small forms to shrink into, therefore letting them take one more hit before being defeated. As a result, the characters lose their advantage over Mario and Luigi and require their regular forms to break Brick Blocks, although they still start the level in their regular forms as they did in prior versions, unlike Mario and Luigi.

Characters return in their Small form after being defeated or falling in a hole or in lava (or if they are in their usual regular forms upon touching lava, they shrink in the bubble to their Small forms) even though they start the level in their regular forms, like how they did in the pre-Fall 2017 versions. However, in the Remix 10 mode, their form is determined based on whether Mario completed the fourth course while Small or Super, similar to Super Mario Advance. For example, if Mario is Small when he completes the fourth course and switches to Peach, she starts off in her Small form, as opposed to her regular form.

As in Super Mario Bros. 2's remakes and Super Mario 3D World, Peach's hair gets shorter when she shrinks into her Small form.

Development
When developing Super Mario Bros., players were originally supposed to start as Super Mario. However, because of the way the camera was zoomed in during development, it made seeing around Mario difficult. After pulling the camera away, Mario appeared smaller, at which point Toshihiko Nakago suggested they make Mario smaller instead of pulling the camera away. The team then decided that Super Mario would shrink into Small Mario if he touches an enemy, but if Small Mario touches an enemy, he'll be defeated instead.

When asked if Shigeru Miyamoto considered letting the players start as Super Mario rather than Small Mario, he responded with "Starting with the small Mario would make players happier when Mario got big later, and it would also give a better impression to players."

Trivia

 * Due to certain glitches in Super Mario Bros. and the Japanese version of Super Mario Bros. Deluxe, it is possible to turn into Fire Mario while remaining Small.