Just because it's habitable doesn't mean there's life there already. Afaik, habitable only means it's about the right temperature, doesn't kill us with the suns radioactive rays thanks to an atmosphere and doesn't crush us like bugs when I stand on it. It should also be able to contain oxygen, again thanks to the atmosphere. Basically that we can be there naked and not die.
Also big =/= huge gravity. They don't actually have anything to do with eachother.
Don't forget the single most important key for life, Water!
Actually, a lot of iron will have more mass than a little bit of iron. So with more mass, means more gravity. You might be think of density which does not = mass. A pound of feathers is not lighter than a pound of lead, but lead is denser.