From the Super Mario Wiki
The Giant Stu is a large Strollin' Stu. It appears in Super Mario Sunshine. The Giant Stu acts exactly like normal Strollin' Stus, and is at the bottom of a Stackin' Stu. When it is defeated, all regular Stus that make up the rest of the Stackin' Stu will also be defeated. They are immune to being turned into a Yoshi Platform.