We are talking about Newtonian Physics here....
As we have said, momentum in archery is a good thing. The formula (in metric)is p=mv (momentum = mass x velocity).
Following the principals of a balistic pendelum, kenetic energy is lost at impact in the creation of heat (frictin loss of hide and flesh, including the conversion of ke to sound, heat, etc. - so forget kinetic energy calculations altogether, and don't use it in our sport - period! The wheelie guys love to sell it because of how impressive the numbers can get, but it means nothing here) but as the laws of physics state that memnetum is conserved (geek note: this is true only at impact where momentum before impact = momentum after impact) so as velocity is decreased, the transfer of momentum is made to the object, and you get penetration.
Hence, more mass of arrow the more momentum, and more penetration.
Sorry, the engineer in me kicks in again....
Clear as mud, eh? This is what you get when I have a good coffee buzz going. Dang Starbucks!