The diameter of Mercury's iron core is about 3/4 the diameter of the planet itself. Why does Mercury have so much iron? Planets that form near the Sun where it is hotter naturally have a greater proportion of elements that remain in the solid form at higher temperatures. So Mercury has a lot of iron but little water. But this effect is not strong enough to fully account for Mercury's huge core. Instead, it is likely that Mercury experienced a giant collision that stripped off most of its rocky mantle. The rocky material was accreted by the other planets, primarily Venus, leaving Mercury with an extreme iron-to-rock ratio. For more on Mercury's composition click here.