Air resistance also increases in proportion to the square of a projectile's speed. So, a fast arrow will lose proportionally more speed over a given distance than a slower arrow launched with an equal amount of KE.

Take a 350 grain arrow at 300 fps and a 700 grain arrow at 212 fps and go to the ballistics program. Both arrows carry 70 fpe from the bow. At 30 yards, the 350 grain arrow has slowed to 285 fps (-5%) and lost 7 fpe (-10%). The 700 grain arrow is going 207 fps and carries 66 fpe, a loss of only 5 fps (-2.4%) and 4 fpe (-5.7%). And the 700 grain arrow is carrying a whopping 31% more momentum at 30 yards than the light arrow.

I assume the resistance in hide, flesh and organs as the projectile passes thru an animal also increases in proportion to the square of the projectile's speed. If the heavy arrow penetrates air with less resistance, then it has to penetrate better in flesh.

Personally, I think far too many people put way too much importance on speed and flat trajectory over typical hunting distances when they should be more concerned about getting their arrow to the animal with as oomph as possible. But then, how deep do you have to bury the arrow in the dirt on the far side of the deer?