Amagon

Amagon (突然！マッチョマン) is a Action/Platformer 2D video game published by Aicom, American Sammy, Vic Tokai released on 1988 for the NES.
Amagon, the most decorated Marine, was assigned a mission by his commander to investigate a strange, monster-infested South Pacific island, from which no man has ever returned alive. With a machine gun as his only weapon, he took off by plane and crash-landed on the island's beachhead.
Amagon, with his unique ability to transform into the huge Megagon, there begins his mission to become first to conquer the island. See and experience the amazing adventures of our hero, Amagon! His rescue ship is waiting at the other side of the island. Good luck!