That's talking about "mini cars" like Smart that are so tiny they actually are compromising safety for weight, not normal sized sedans.
For example:
> The second factor is vehicle size, specifically the distance from the front of a vehicle to its occupant compartment. The longer this is, the lower the forces on the occupants.
A compact car and a crossover are typically about the same length. A "mini car" is shorter than that, which is what compromises its safety. And unlike making the vehicle heavier, making the nose longer doesn't harm the occupants of the other vehicle in a two party collision, and does actually help you in a collision with a massive stationary object (e.g. overpass) whereas weight in that case does almost nothing.