Why is Mercury the smallest major planet in the solar system?

Mercury is the smallest major planet in the solar system, with a radius of approximately 2,440 km and a mass of $3.30\times {10}^{23}$ kg. While its small size is a defining characteristic, the specific reasons for its limited mass and volume remain a subject of scientific study rather than a single settled fact.1234

One prominent theory suggests that during its early formation, Mercury may have experienced a giant collision with another body, which stripped away much of its outer silicate crust and mantle, leaving behind a core-dominated planet. Other hypotheses suggest that its composition was influenced by the extreme conditions of the early solar nebula, such as the preferential vaporization of lighter silicates due to the intense heat near the Sun or the sorting of heavier metallic particles through aerodynamic drag.4

Regardless of its origin, Mercury is uniquely dense for its size, with an iron-rich core that accounts for nearly two-thirds of its mass and about 85 percent of its radius. This high density distinguishes it from other bodies of similar size, such as the Moon.13