A bigger, heavier vehicle provides better crash protection than a smaller, lighter one, assuming no other differences. Both size and weight affect the forces experienced by vehicle occupants during a crash. The magnitude of those forces is directly related to the risk of injury.
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 beyond weight, SUVs and pickups used to pose a bigger threat to car occupants because their energy-absorbing structures didn’t line up with those of cars. Voluntary design changes by automakers have largely solved this problem.
Fuel economy can be improved without sacrificing safety. Electric vehicles, hybrids, more efficient internal combustion engines and other technological advances can raise fuel efficiency without affecting occupant protection. Taking a small amount of weight off the heaviest vehicles while keeping size steady also improves fleetwide fuel economy without safety tradeoffs.