When Kirby inhales the miniature enemy, Minny, he too gets downsized to the size of a button. It is notable for being the only ability, besides Sleep, which actually hinders Kirby's combat skills rather than improve it.
Mini is only really useful to retrieve several of the game's Treasure Chests. This is because Mini has the ability to allow Kirby to enter small cracks and crevices. The only downside of the ability is that Kirby is very vulnerable when he is such a small size, and has no way of defending himself against any foe or calling his friends with the cell phone. When Kirby goes through a door with this ability, he loses it. Even though this ability does not help with defense against enemies, it is a rare Copy Ability and is near-impossible to find.
Kirby is exactly the same as normal, but without the ability to slide, inhale, and float in the air. He can, however, jump and enter small areas.
Shrink down to tiny size so you can fit through tight passageways.
In Kirby: Squeak Squad, there are Kirby Bubbles that Kirby can find with a small Kirby-like figurine inside it. While it does not give him a power, he can get a 1-up if he mixes three of them together.
Although not an ability, Mini Kirby does make an appearance in the anime in which Kirby is shrunken down by a newly purchased Microtizer from Nightmare Enterprises.
In Kirby's Adventure, unused sprites for a smaller Kirby can be found. This set even comes with basic movement and an "ability presentation" sprite where Kirby shrinks. Masahiro Sakurai confirmed in a 1993 interview that an ability causing Kirby to shrink was indeed planned to appear in Kirby's Adventure. It is possible that the developers of Kirby & the Amazing Mirror were aware of this scrapped ability from the interview in question, from reading old HAL Laboratory development notes, or from seeing the leftover sprites in Kirby's Adventure's code.