Mars 'remains in embryonic state' (BBC)

By Jennifer Carpenter
Science reporter, BBC News

Mars formed in record time, growing to its present size in a mere three million years, much quicker than scientists previously thought.

Its rapid formation could explain why the Red Planet is about one tenth the mass of Earth.

The study supports a 20-year-old theory that Mars remained small because it avoided collisions with planetary building material.
***
By studying the chemical composition of meteorites, geochemist Dr Nicholas Dauphas of the University of Chicago in Illinois and Dr Ali Pourmand of the University of Miami in Florida joined forces to try to confirm this.

By measuring the concentration of elements Thorium and Hafnium in 44 space-rocks Dr Pourmand and Dauphas have come up with the most precise estimate of the time it took Mars to form.
***
more: http://www.bbc.co.uk/news/science-environment-13567381

Using Hf-W-Th ratios to support the hypothesis is something I'm not familiar with, and the abstract doesn't give enough detail to really figure out their reasoning. For $32 to read the full article, I guess I can remain mystified. Oh well...

if the asteroid belt had somehow smashed into it?

Obviously one way to make Mars more habitable would be to increase its mass. We don't have anywhere near the tech to do it, but one day we probably would. Although I think we could terraform it today for intermediate periods of time (and ungodly amounts of money that make the idea currently unthinkable). We'd still have to replenish an atmosphere that would be blown away by the solar winds or find a way to create a planetary magnetic field, but the former would be doable.

Not going to happen in the next 300 years but it's foreseeable.