- Luffy uses a new attack called “Gomu Gomu no Gigant” turning his entire body into giant. He looks like a God.
This *could* explain the big hat? I don't know, but it occurs to me that the size of a hat is typically meant for someone in a base form.
I would have to argue the point that the original person with this power was bigger than an average human. Otherwise, I have to assume something weird like Joyboy transformed into his Gigant mode and then died, but his hat never shrunk back to its normal size. So, we ended up having a big hat that isn't proportional to any specific person.
I would say that's a stupid line of reasoning, though. One of two things happened here (1) Joyboy was a giant originally OR (2) Joyboy was always a small person, but he used an attack that made his body gigantic, and as a result his hat too.
You tell me which one is more believable.