bowtech11, its all a matter of how hard it is to stop. A heavier arrow, even tho it is moving at a slower speed, is harder to stop. A lot of guys like to use the ping pong ball and golf ball analogy, although its always open to argument cause no one shoots arrows that weigh as little as a ping pong ball. But which would you rather be hit with? A ping pong ball moving at 500fps, or a golf ball moving at 170fps? (I used 170fps cause thats typical traditional, recurve/longbow, arrow speed). I think that ping pong ball is going to sting like hell, but the golf ball is probably going to break a rib, in spite of the fact it is going so much slower. Follow me?
Here's a helpful chart
http://archeryreport.com/2009/12/arrow-momentum-quick-reference-chart/
Now if we take my bow which shoots a 300gr arrow at 335fps, we get a momentum reading of .445
Take the same bow and shoot a 450 gr arrow at 270fps, and we get a momentum reading of .539
So in spite of the slower speed, the heavier arrow has more forward momentum and will therefore be harder to stop. It only makes sense that if the arrow is harder to stop, it is going to penetrate farther than the lighter arrow with less momentum.
All this can be further enhanced by broadhead design and sharpness. You could take the very same fixed blade broadhead, on the very same arrow, one dull, one sharp and the sharper broadhead will out penetrate the dull one everytime.