From the Super Mario Wiki
In New Super Mario Bros. Wii, they appear only in World 7-6 and in New Super Mario Bros. U, they appear only in Soda Jungle level Flight of the Para-Beetles, along with its smaller relatives. They act as platforms for Mario and company to jump on to get across the level. Despite being three times the size of a normal Para-Beetle, the Heavy Para-Beetle cannot stand the weight of any character and begins to descend when stepped on, unlike their smaller cousins, who fly higher when stepped on. Like normal Para-Beetles, the Heavy Para-Beetles can be defeated only by running into it with a Starman. They can not hurt the character while he is standing on its shell, but its exposed head and underside can hurt the character. Like the other Para-Beetles, if the player jumps on a certain amount, 1-Ups fall down to the player, or each player depending on the amount of people playing.