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. It cannot be defeated by using Yoshi Juice and turning it into an Enemy Platform, since they are immune to this.
A giant, beta Strollin' Stu, even larger than a Giant Stu, was to have appeared in an unused plaza, but was taken out of the game for unknown reasons.