Super Mushroom

The Super Mushroom (Mushroom for short) is a power-up/collectible in Super Luigi Country. It can be used by Luigi and various enemies alike, touching it will grant the user to turn big in size. It is also a reoccurring power-up in the Super Mario series.

Story
'WARNING! Spoilers for Super Luigi Country will be found. Read with caution.'

Several enemies are implied to have eaten a Super Mushroom which would explain the abnormal size. The Super Mushroom plays a vital part in three boss fights, Bidoof's, Captain Olimar's, and Dixie Kong's fights.

Level 2
It could be implied that the Waddlewing in this level ate a Mushroom which made it bigger.

Bidoof Boss Fight
It could be implied that Bidoof ate a Super Mushroom which made him bigger and much more slower than usual.

Level 13
It could be implied that the Cheep Cheep in this level ate a Super Mushroom and made itself big.

Level 14
It could be implied that Popo ate a Super Mushroom which made him big, though it is unknown how his hammer got big as well.

Captain Olimar Boss Fight
Since Kamek made you small with his magic, you have to get to the top of Olimar's helmet to get the Mushroom and grow back to normal size, this is the only time the player has to get the Mushroom.

Dixie Kong Boss Fight
The cutscene that plays before you enter the 2nd phase of Dixie's fight shows her obtaining a Super Mushroom, she then tries to chase you down, eventually you defeat her and knock the Mushroom out of her.