Why Do We Believe Mars Is So Small

Why do we believe Mars is so small?

A) The material out of which a larger planet might have formed was already incorporated into Earth and Venus.
B) The material out of which a larger planet might have formed was scattered by the migration of the Jovian planets.
C) Tidal forces from Jupiter prevented material here from coalescing into a stable planet.
D) Like Mercury and perhaps Venus, it suffered a major collision in the early history of the solar system.