Neither -- it is simply more convenient and more efficient that way. By filling, you will be getting more overall vapor per charged battery. The battery will provide the same heat regardless if it is loaded or empty. Therefore, any part of the unit which is hot but not loaded is wasted heat (wasted battery charge). You might as well load it fully and get the full benefit of the battery energy. Also, by under filling, the vapor density (the ratio of vapor to air) will be somewhat less than that identified as optimal by Magic-Flight's research.
Since it can be arranged that the Box is only hot when you are actually drawing from it, you get the benefit of having the vapor dispensed only when you want it. When not heating, you might as well think of the Box as "storing" a very small amount of material. Because the Box is able to heat up nearly instantly (no waiting) it is convenient to use. Because the Box is also able to cool off again nearly instantly, it does not waste any vapor either -- this is one of the things that makes this vaporizer efficient.