The problem with this statement, Sixty8, is that physics is almost entirely scalable. Let me explain what I mean with a little analogy.
Take 2 bowling balls. One is 6 inches across and weighs 10 pounds. The other is a foot across and weighs 20 pounds. Drop them both off a building. They will both fall at the same speed, since the bigger ball has more air resistance pushing up, but more weight pushing down. When they hit the ground (let's say we didn't drop them very far), the bigger one will shatter 2x as much as the small one, since it has 2x the kinetic energy to disperse into the ground. So, if the bottom 2 inches of the small one crack off when it hits the ground, the bottom 4 inches of the big one will. So you will be left with two basically identically damaged bowling balls, one twice the size of the other.
So, applying this to cars in accidents. Weight does not always help you if not applied to the right areas of the car. A 4500 pound Benz would have to crush twice as much material as a 2250 pound Metro to stop, assuming they were going the same speed and crashed into the same type of material. If they both crashed into a tree for example, the Benz would do a lot more damage to the tree. If they crashed into a solid wall however, the Benz had better have a hood section that's twice as long as the Metro, with twice as much weight in it, or bye bye passenger section.
If they were to crash into each other, however, you would be right. Say the Benz is going west and the Metro is going east (same speed). The whole mess will still be going somewhat west after the accident, meaning the Metro decelerated to a stop, then accelerated the other way (more force than just stopping) while the Benz didn't quite stop (less force...).
So, having a bigger car won't always save you, but when other people are involved, it usually helps.