The World Government believes that the large Straw Hat is Uranus, one of the 3 Ancient Weapons
Over 800 years ago, the leader of the Ancient Kingdom was a giant who wore a Straw Hat. The people of the Ancient Kingdom were the original clan of D., and their leader who for simplicity’s sake will be called “Leader” was one of the early people who possessed the will of D. He was the man most feared by the 20 Kingdoms because of a strange ability he possessed: He had the power to create an ally out of anyone, friend or foe, and amass an army in mere days. That power was called “Uranus” by the 20 kingdoms, yet none of them knew why he possessed it or how it worked.
Knowing that they’d soon be defeated, the Ancient Kingdom decided to pass their history down to future generations via the Poneglyphs.
In addition to that, they decided they needed to hide their 3 Ancient Weapons. When it came to Uranus, the Leader devised a plan that would not only fool the World Government, but would at the same time shift their attention onto a useless hat made of straw, when the real threat and the real power of Uranus (the Leader himself) was killed instead. How he managed to fool them into believing that the large Straw Hat itself was Uranus is beyond me, but it isn’t important in this discussion. He might have fed it a Devil Fruit or performed an action on it that made it seem powerful, who knows.
As for the small Straw Hat that was newly made by him, he passed it down to a young kid from the D clan and had him escape before their kingdom’s annihilation. He was to travel the world and amass enough power to take those 20 kingdoms down, but as he reached an old age and realized that it just wasn’t possible, he passed his hat down to a young member of his crew along with his hopes and dreams. And just like that, the cycle began.
The important thing to take note of is that the World Government acquired that large Straw Hat and stored it in what would later become the Holy Land of Mary Geoise. For the longest time, they believed that they had Uranus, one of the 3 Ancient Weapons stored in their vault, and even though they didn’t know how to operate it, just the fact that they had possessed it was good enough, believing that there would come a day in which they were able to wield it.
So ironically, a large hat made of straw was passed down through generations within the World Government, each generation unable to make it work, while the real threat, the will of D symbolized by a smaller hat made of straw, was passed down through generations in the shadows, each generation growing stronger and closer to defeating the World Government.
Why the small Straw Hat went unnoticed by the World Government for so long
The reason behind the small Straw Hat’s creation was merely symbolic. Just like the large one, it didn’t possess any hidden power and was nothing special at all. It was merely a symbol of the will of D, the will of the Ancient Kingdom, and the 20 Kingdoms who later became the World Government never paid any attention to it because they had no reason to.
Hats made of straw aren’t a rarity in the world of One Piece, at least I’d like to believe that, and the owners of the Straw Hat through the different generations had never managed to accomplish anything major, always failing and passing it down to future generations. I believe Roger was the first D to go far enough to be noticed, and even then, he had already passed his hat down to Shanks before being a big enough threat, and Shanks himself passed his hat down to Luffy before becoming an Emperor. This is why neither Roger nor Shanks are known as Straw Hat Roger or Straw Hat Shanks; even though some people remember them wearing it, they weren’t a big enough deal when they did.
As for why the Leader created it in the first place, he might have intended for it to be some sort of message to the 20 kingdoms. After he and his ancient kingdom had died, he might have wanted the 20 kingdoms to witness their downfall at the hands of a man in a Straw Hat, a way of telling them that they had failed and let a D escape, but that obviously never happened and the Straw Hat itself became more of a symbol of D rather than a device aimed at irony and revenge.
Luffy is Uranus
Or rather, he’s another Uranus, with the first and only one having been the Leader. I talked earlier about what power Uranus was capable of. I believe that the Leader of the ancient kingdom possessed the same power that Luffy currently possesses; the power that Mihawk himself praised as being “the most dangerous ability in this entire ocean”.
Uranus, or rather Luffy, possesses the power to turn foes and distant people into allies willing to sacrifice their lives for his ideals. We’ve seen this multiple times in the past, and it culminated itself in the formation of the Grand Fleet in Dressrosa.
Think about it. Pluton is a large ship capable of massive destruction. Poseidon is a mermaid able to talk to and command Sea Kings. You’ve got a war ship and massive beasts as two weapons of mass destruction. All that’s really left is an army, which is what Uranus is. And similar to how Poseidon turned out to be a living thing, Uranus will turn out to be none other than Luffy.
“You’re not the man Roger is waiting for”
Is what Whitebeard told Blackbeard before he died to him. The major difference between Luffy and Blackbeard, other than the fact that one is good and one is evil, is the fact that Luffy possesses the one ability able to take the World Government down: His heart and ideals are so pure that even foes eventually get moved by them and ally themselves with him. Blackbeard, on the other hand, only knows how to backstab, kill and use people until he got what he needed. He’s not the man Roger or any of the people of the D clan are waiting for because no matter how strong he gets or how many Devil Fruits he amasses, he doesn’t have an army that believes in his ideals, an army ready to sacrifice itself in the blink of an eye to save his life, and he isn’t willing to do the same for them either, unlike Luffy who’s shown multiple times how far he’d go to save a friend.
The World Government’s realization
Back to the World Government’s large Strawhat, the middle finger in disguise they took off of the Leader of the Ancient Kingdom. For over 800 years, they believed that they’d possessed Uranus. They had no idea how to operate it and spent centuries trying to decipher it. They had it frozen and kept in a well guarded place for that reason.
Then a boy shows up wearing a small Straw Hat. He stirs up some trouble in Alabasta and a few other islands, but he’s nothing dangerous. He enters the New World a couple of years later, goes down to Fishman Island and takes out Doflamingo, but again, he’s merely one of many troublemakers, and people like Kidd and Blackbeard are far more dangerous because of what they’ve done in regards to murdering people. Then he builds himself an entire fleet in a couple of days, infiltrates an Emperor’s territory, allies himself with the Germa kingdom and Bege’s crew, takes said emperor’s crew member (Jinbe), defeats her first commander, trashes her main island and runs away nearly unscathed. Now the World Government has taken notice and tripled his bounty to 1.5 Billion, something that was believed to be almost impossible after a certain threshold.
And now the similarity between the 2 Straw Hats starts to take shape. Before all of that, they were 2 separate and unrelated entities, but it’s different now. Seeing this all take shape, a mysterious and important figure from Mariejois takes Luffy’s bounty and I’m guessing Roger’s as well, goes down to the vault that holds Uranus, compares the bounties to the large Straw Hat, and it clicks.
“The hat is useless, we’ve been fed lies through generations. The real threat is the kid with the small Straw Hat”.
*Theory by Steins-Fate