vehicle size does not directly translate to safety, other than "perceived safety" for some people... ie: people "feel" safer in a larger vehicle
the design and build quality of a vehicle have far more to do with actual safety than size does
what that video is showing is that even though the 2009 Malibu is a smaller and lighter weight car, it fares better in this accident demonstration thanks to its crumple zones and safety cell cabin design
whether a smaller or larger car would have fared better or worse in this test can not be determined based strictly on vehicle size alone, again factors such as design and build quality have far greater of an effect