That still don't really tell me why or whats different between the two harnesses. Is it because HIDs need more amps and the HD harness is a heavier wire? because simply uping wiring size won't get you anymore power, 12v is 12v
12V is not simply 12V. You see the factory headlight harness draws its power for the headlights through the headlight switch, through ~16 gauge wire. By the time the power reaches the headlights, there is a substantial drop in voltage at the headlights. So by the time the power reaches the HID's (or halogens for that matter) they are only seeing ~9-10 volts a lot of times instead of 13.8V+. HID's draw a spike in amperage upon start up, which is more amperage then then the OE headlight harness can supply to both headlights. Which is why you'll get only one ballast igniting, or flickering issues. So the HD Harness is not only upgrading the wire size, its also switching the power source for the headlights. So instead of drawing there power from the headlight switch, the headlights will draw all there power directly from the battery and NOT the failure prone headlight switch.
With a HD Harness installed, the power for the headlights is being supplied directly from the battery. Thus providing your headlights with full battery voltage of 13.8V+ constantly. By upgrading the size of wire, your ensuring that there is next to no voltage drop. So now with a HD Harness your headlights are receiving 13.8V+ instead of the ~9-10 volts (or less) that the OE harness was supplying them. As well as being able to easily supply proper amperage to the headlight bulbs or ballasts when they need increased amperage.
Upgrading wire gauge is essentially like increasing the size of a water hose. The larger the diameter of the hose, the more water that can flow through it in a given period of time.
With the OE harness the HID's will starve for power, this is why you get mis fires and flickering and short life's of ballasts and the like. The HD Harness takes care of these issues by ensuring that they get full voltage allowing them to run at 100%. You can really see the difference that the full 13.8V+ makes with halogen bulbs in particular. If you take a look at your headlights prior to installing the HD Harness, and compare it to after you install the HD Harness; its a night and day difference. The headlights are visibly brighter, being they are not starving for power and are now receiving 13.8V+.