A heavier arrow will always penetrate better than a light one when shot out of the same bow because it carries more momentum. KE is only part of the figure and not the most important part in my opinion.

That being said using a light arrow really depends on your set up. Most people shoot enough energy with modern compounds that it really isn't an issue on deer at normal ranges. Now when you are talking lighter set ups like mine with a short draw length and lower draw weight it starts to make a difference. For someone like me it pays to play around and find a good midway point. If I go too light I won't get pass throughs, if I go too heavy I start to really lose speed, I'm talking under 200 fps.

However for someone shooting a newer bow with more than 60 lbs and 28 inches or more of draw length you could darn near shoot any weight arrow you wanted if you used a sharp head and picked your shots well.

My opinion anyway.

Paul