It depends on the age and the girl but I'm sure many know the effect they have and there's nothing wrong with that.

People like to feel admired and sexy. We live in a world that celebrates beauty, just look at kids TV with all the pretty blond girls. People instinctively want to be noticed and admired and so skimpy shorts are both practical and a way to draw some attention.

So don't feel guilty, you're giving a compliment and you're just admiring, as long as you don't make her feel uncomfortable or threatened which you won't from just admiring from afar.