What you are seeing is that when you increase your arrow weight to much, you are dropping to much speed. Theres actually a maximum arrow weight that will give you the maximum amount of KE. Use the KE calculator I posted yesterday to find this. Many times people think the heavier your arrow is, the more penetration you will get. WRONG. Lets look at this. If you shoot an arrow at exactly 350 grains, you might get 70 ft/lbs KE at 300 fps. But if you throw a HEAVY log of an arrow on there that weighs 650 grains, I'm going to see that my FPS has dropped to ABOUT 200. And my KE has dropped to about 58 ft/lbs. BUT WAIT, I thought the heavier the arrow, the more KE. That is a misconception.

And this is why. You have added so much weight to your arrow that you now have slowed it down so tremendously that the arrow energy has actually been decreased. (Check it in the KE formula if you need) Since the speed has dropped so much you have also lost KE. Its kind of like a teeter totter. When one side increases (arrow weight) the other side must decrease (arrow speed) and thus the KE is affected through these two things. If you think of the arrow weight as your variable you will notice that KE and your fps are its dependents. The KE and FPS are DEPENDENT on the arrows weight.

Lets think of this in a real world example. If I take a bowling ball and roll it into you at 5 mph and then I take a baseball and throw it at you at 50 mph which is going to hurt more?

For the guy who said that the KE may be lower, but the momentum will be higher is actually not exactly right. While there is some truth to his arguement, KE and momentum are so closely related and almost identical in formulas that their difference doesnt translate into real world use.
Bottom line is KE and momentum are almost identical scientific properties in a reality sense.

To sum all of this up in a snetence or two this is the bottom line. If you add to much weight to your arrow, the fps slows down so much that it negatively affects your KE. Likewise if you hunting arrow is to light you will gain massive speed, but will be too light to have high KE numbers.