Justin, the foam is designed to stop arrows. It does so by trying to re-close after the arrow penetrates. It took longer to get a grab on the smaller diameter arrows.

In real world, deer and living tissue is under tension that tends to pull apart when penetrated versus close back up. I doubt you would see a large enough difference to warrant diameter to be a primary consideration in shaft selection.

For those of you slinging two blade cut to tip heads such as Magnus, who stated you've yet to not achieve a total pass through on deer, how many deer have you shot and how many shots encountered bone, rib or shoulder? Would love to hear more on these real world hits.