Did Luffy Really Say “Power Isn't Determined by Your Size, but by the Size of Your Heart and Dreams”?

No, as far as anyone can show. "Power isn't determined by your size, but by the size of your heart and dreams" has been attributed to Monkey D. Luffy since at least 2017, has been tweeted by a phone company and by a ministry of defense, and in all that time nobody sharing it has pointed at a chapter, an episode, a film scene or a minute mark. That is the shape of a line that was written for him rather than by him.

The verdict

Fabricated. Not because it is absent from one place, but because it is absent from every place a real line shows up. A search of all of Wikiquote for "size of your heart" returns two hits, from The Munsters and Alias, and none from One Piece. Screen Rant's ranking of Luffy's ten best lines and CBR's list of thirty do not include it. Game Rant's list of fifteen does, as a title-cased heading, and the paragraph under the heading describes Luffy's attitude to big opponents in general and names no scene. CBR's only other appearance of the sentence is a news item about a government account tweeting it, which reports the tweet and vouches for nothing. On the Japanese side, where fan catalogs list Luffy's lines by arc in exhaustive detail, three natural phrasings of the thought turn up no line about strength being measured by the size of a heart or a dream. A line that exists in English motivational posts and in no Japanese catalog is a line that was written in English.

The film story

When a source is claimed at all, it is "the movies," and one quote site specifies Strong World, the tenth film. Nobody gives a scene. The claim is worth taking seriously for one reason: this site has been here before. "If you don't take risks, you can't create a future" was shared for years without a source, looked like a poster, and turned out to be a real line from the second film in a subtitle rendering. So a film attribution is not nothing. But that line was placed by people who had watched the film and could name the boy it was said to. This one comes with a film title and no moment, in a wording that reads like the caption on a gym poster, and on the question pages where fans ask where it is from, no one answers. Strong World's English release has not been checked line by line for this page, and the record says so; if the line is in it, the verdict changes the day someone shows the scene.

Why it stuck

It fits a true thing about the character. Luffy is small and fights giants, literally and otherwise, and the series keeps arranging a small frame against an enormous one. Game Rant's paragraph is right about that, which is the trouble: the sentence is a fair description of Luffy that has been reworded as a line of dialogue. The real Luffy does not explain his own theme. When he tells Coby what he wants in the first episode, or explains that he does not want to conquer anything, only to be the freest man on the sea, the lines are declarations, not aphorisms about what power is. Luffy talks about what he is going to do.

What he actually says

His verified lines are on our Luffy page, each with its source, and they are shorter and less tidy than the poster wording. Two other lines this database has checked under his name are worth reading beside this one: "forgetting is like a wound", which is fabricated on the same evidence, and "when do you think people die", which is real and belongs to Dr. Hiriluk. The most famous character in the series attracts sentences the way a hull attracts barnacles.

How we checked

The verification process asks for a primary or editorial source printing the line inside a sentence. Here there is one heading, on one publisher, with no scene beneath it, and nothing else at any tier. Wikiquote was searched in full, the three editorial Luffy rankings were read, and the Japanese fan record was searched under the phrasings the sentence would naturally take. It enters the fact-check database as fabricated, with the film caveat above written into the record, and the contact page is where to send the scene if it exists.