It has been theorized that both Earth and Venus were subjected to numerous impacts by large bodies, which increased their size and mass considerably, but Mars avoided the big collisions.
Perhaps the largest, or the last, collision to hit Earth is estimated to be about the size of Mars. Venus is about 95% the size of Earth, Mars only about 50%
Quibble, Mars is only 1/9th Earth’s mass. While Mars has been continously refaced, and has even had at least a couple (because the impact craters remain visible) of impactors so large that, if there had been an ecosystem, it would have been wiped out.
Re: a Mars-sized impactor on the Earth—
http://www.freerepublic.com/focus/chat/1234919/posts
whoops, I got distracted and didn’t finish that sentence — Mars has been continuously refaced, but hasn’t picked up much mass in the process.