Giant Bowser

Super Bowser is an enormous form Bowser takes in the New Super Mario Bros. series. He is the final boss of every game in the series except for the original New Super Mario Bros. (though he did appear slightly larger during the aforementioned game's final battle). The form comes about when either Kamek or the Koopalings use their magic wands to resurrect a defeated Bowser.

Super Bowser should not be confused with other occasions outside the New Super Mario Bros. series in which Bowser's size increases.

New Super Mario Bros. Wii
At the end of in New Super Mario Bros. Wii, Mario encounters Bowser on a bridge. Mario gets past Bowser and hits a switch that destroys the bridge, sending Bowser plummeting into the lava below. Kamek appears, however, and spreads magic dust into the lava, which begins to rumble and boil. When nothing happens, Kamek looks quizzically at the lava, when Super Bowser bursts out of the pool, knocking Kamek away, and roars at Mario, initiating the final boss battle.

In order to defeat Super Bowser, Mario must make his way through a labyrinth, allowing Super Bowser's fireballs and claw swipes to destroy structures blocking his path. Some ? Blocks, Coins and Goombas appear in this section. Eventually, Mario will reach a new section composed of floating platform. A Propeller Mushroom may be collected at the beginning of the section. Bowser will use a new attack here, pounding the lava to send waves at Mario. Mario will soon reach another, larger switch, which sits below the real Princess Peach's cage. Hitting the switch will destroy the floor below Super Bowser, defeating him and completing the game.

In the post-credits epilogue, Bowser is seen returned to normal size, being helped by the Koopalings, only for the castle to fall on them all.

New Super Mario Bros. 2
At the end of World 6 in New Super Mario Bros. 2, after defeating Bowser in his regular form, the Koopalings fly in in the Koopa Clown Car and use magic on him, causing him to turn into Super Bowser. To defeat him, Mario must jump from one unstable platform to another until he reaches a big ! Switch at the top of the tower. After Mario presses it, Super Bowser is defeated and Princess Peach is freed.