The formula for KE is speed x speed x weight / 450240. As you can see, it is directly affected by arrow weight and speed, so, the idea is to get a heavy arrow to fly fast. By shooting a lighter arrow, you will gain more speed, but there is a point where the arrow is too light to gain any KE. The same is true if you shoot a heavy arrow, you will lose speed, but, there is a point where the curve will reach it's apex, and the KE will be at its highest before it starts to drop off again. So, the balance of the two is important.
Another factor to consider: A heavy arrow will have more momentum, which some ppl believe is even better than KE. A light arrow will lose momentum (slow down) much faster than will a heavier arrow. So, the heavier arrow will have more downrange momentum, which is a good thing.
I downloaded Easton's arrow chart, they have a KE calculator, so, if you know how much any given arrow weighs, and the speed your bow shoots it, (or at least a good guess), you can play with the numbers to get the happy medium you are looking for. Wow, I think I confused myself......hope this helps.