Early on in the solar system's history, the Sun experiences a large amount of solar activity. This activity had two effects on our solar system. Mercury is pushed out of its orbit, and passes fairly close to Venus. Mercury goes on to collide with the Moon, increasing its mass - while adding atmosphere onto the new moon. Ths also pushed out the moon futher out from Earth, thus helping to negate many of the harsher effects of a larger moon.
Around 4 Billion years ago, another early proto-planet collies with Mars, at such an angle that it gets pushed out of its orbit. As it is heading toward the sun, it is caught by Venus and begins to cause huge eruptions on the planet, as well as gravity fluctuations. This causes nearly proto-planets to crash onto Mars increasing not its size, but density.