Stealth Force,

Does the 400 grain arrow have a smaller diameter than the 600 grain arrow? If so, there is the reason it penetrates more. The smaller diameter arrow and point combo will obviously penetrate more. I have found that even skinny arrows (a/c/c and such) with a larger point will penetrate very well because the only friction with the target is the point. Another factor in penetration is the arrows finish.

If the arrows were exactly identical other than weight, then you have a point.

X-Ring