The thing 'size' gets you is crumple zones. A bigger vehicle has a lot more material that can be engineered to deform and absorb the energy of the impact
Not the only thing. Recall your high school physics. Crank through the basic motion equations for inelastic equations. The more massive object experiences less change in momentum. All things being equal, passengers of the more massive vehicle are going to experience lower g-forces.
Not to be overly pedantic, but I intentionally said "size" not "mass". As car manufacturers increasingly use aluminum and various composites in place of steel, bigger isn't always heavier.
g-force depends on acceleration (or in this case, deceleration) by definition. Deceleration is change in speed over time for it to occur. The whole purpose of the crumple zones is to maximize that time and thus reducing g-force.
Not the only thing. Recall your high school physics. Crank through the basic motion equations for inelastic equations. The more massive object experiences less change in momentum. All things being equal, passengers of the more massive vehicle are going to experience lower g-forces.