Density is measured by mass per volume. Mercury's core is 40% iron. Its size is the smallest in the solar system. If so Mercury should be the densest planet, right? Unfortunately, no. That goes to Earth(5.515 g/cm3). Considering their gravitational compression, Mercury came in second (5.427 g/cm3). Just slightly below Earth's. If Mercury gain enough weight, then they would have the same density!

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Source: https://solarsystem.nasa.gov/planets/mercury/by-the-numbers/