A bigger, heavier vehicle provides better crash protection than a smaller, lighter one, assuming no other differences. The longer distance from the front of vehicle to the occupant compartment in larger vehicles offers better protection in frontal crashes. Heavier vehicles also tend to continue moving forward in crashes with lighter vehicles and other obstacles, so the people inside them are subject to less force.
Large vehicles aren't as big a threat to people in small vehicles as they used to be. A lighter vehicle will always be at a disadvantage in a collision with a heavier vehicle. But in recent years automakers have reduced the threat SUVs and pickups pose by more closely aligning their energy-absorbing structures with those of cars.
Fuel economy can be improved without sacrificing safety. Various technologies can raise fuel efficiency without reducing the vehicle’s weight. Manufacturers can also improve fleetwide fuel economy by taking a small amount of weight off their heaviest vehicles without significant safety tradeoffs.