I think someone (can't remember who at the moment) made a really good point that with Kate buying off the rack, she needs to take into consideration that while her size might be good for off the rack clothing, her height puts her at a disadvantage in terms of length.
Personally I don't think the dress was that short but as evidenced by her trying to exit the car, it probably wouldn't hurt to add a couple of inches to her dresses. And really, you can't do that off the rack. Its liek if you are a little hippy, you want to purchase a 14W for example, but you are short, so it hangs off your lengthwise but you can't do a Petitie size because it takes into consideration your height and not your width!