During a press tour for Age of Ultron, the Avengers actors were shown photos of each other's arms, and they had to identify which cast member they belonged to.

But everyone was surprised by the development of Robert Downey Jr’s biceps…

The picture of his arm is taken from a scene in Iron Man 2, where Tony is creating a new core for his reactor. Although Robert is very lean and slender, he does look really big at one particular angle, which is why the photo is so surprising.

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