Instead of thinking "capacity" by itself, thing of "capacity for the given space" or rather "density". So Sodium batteries can be equal capacity as Lithium, but the equivalent capacity Sodium battery will be significantly larger. In applications like storing overproduced wind or solar electricity for use later, we don't care how big the battery is. However, it a moving vehicle where every square centimeter and every kg changes the performance of the car, those density differences can have a real impact.