From the Super Mario Wiki
A Red Apple is an item in Super Paper Mario. Found on a tree in Overthere Stair, it restores five HP and cures Poison. If the player tries feeding Peach this apple, she wakes up from the deep sleep caused by the Golden Apple and wonders what's happening. Then, suddenly, she grows huge, about the size of the tree. A few seconds later the effect wears off, Peach shrinks and goes back to sleep. Later in the game, when the player has to feed a cloud named Cyrrus a Red Apple to make him grow huge, after which point the player can bounce on him to get to a higher area of the Overthere.
The Red Apple's power may be a reference to the size increase granted by Super Mushrooms, which are also red, just as how the Blue Apple that makes Peach shrink is blue like a Mini Mushroom.