Small Peach is the weakest form of Peach in her playable appearances in the main Super Mario series. Super Peach becomes Small Peach after being hit by an enemy or an obstacle. This form does not have any special abilities whatsoever, nor does it require any power-ups to assume this form. Taking damage will immediately cause her to lose a life.
Although Peach normally starts in her Super form in the original and All-Stars remaster of Super Mario Bros. 2, Super Mario 3D World, and post-Fall 2017 versions of Super Mario Run, she starts in her Small form in Super Mario Advance instead, rather than her Super form.
In the original NES version of Super Mario Bros. 2, when Peach shrinks into her Small form, only her lower half shrinks, while her upper half (including her hair) remains intact. In the remakes of Super Mario Bros. 2, this is changed so that her entire body shrinks. As a result Peach's hair gets shorter to be proportionate with her body size. This would later be carried over to Super Mario 3D World and post-Fall 2017 versions of Super Mario Run.
In earlier versions of Super Mario Run prior to the Fall 2017 update, Peach did not have a Small form to shrink into when being hit unlike in Super Mario Bros. 2 and Super Mario 3D World. Instead, she was defeated in one hit and remained in her normal usual form. In versions as of the Fall 2017 update, she has been given a Small form, therefore letting her take one more hit before being defeated just like in said titles. As a result Peach loses her advantage over Mario and Luigi and now requires her normal usual form to break Brick Blocks just like in Super Mario 3D World. Unlike in Super Mario Bros. 2 and Super Mario 3D World, Peach respawns as Small Peach after being defeated or falling in pits or lava similarly to Super Mario Advance, even though she starts the level as Super Peach,unlike Mario and Luigi who always start in their Small forms. In the Remix 10 mode, her form is determined based on whether Mario completed the fourth course while Small or Super before switching to her. For example, if Mario is Small after completion of the fourth course, she will then start as Small Peach even though Super Peach is the state that she is in when being kidnapped and her starting form in the other modes.