I suspect that the anecdotal observations you've made are more coincidental than correlational. To have seen 60-80 head of game fall to 6mm and .257" caliber bullets with no issues other than a small handful of exceptions tells me that it may have more to do with other factors than the caliber of the bullet. Either inadequate bullet expansion or perhaps different vitality among individual animals. For example, caribou are not especially hard to lay down but I witnessed one bull take 3 rounds of 225gr SP from a .338 Win Mag through the shoulders and lungs from different angles before going down, and he needed a 4th round to finish him upon approach. After the first shot the hunter thought he had missed because the bull gave no reaction to the hit, but I saw it through my bino. I told him to hit the bull again. And again. And finally a finisher was needed. It was an anomaly and definitely doesn't mean that .338" bullets are mediocre performers on caribou. I've also witnessed bull moose, which often take a while to drop after being shot, go down instantly when hit through the shoulder with a 7-08, but I've also seen them stand around after being hit in the shoulder with a .300 Win Mag. I saw a big WT doe get hit by a 105 AM from the .243AI, and at the shot it looked like a can of tomato soup exploded out from the deer's hide onto the snow. When we walked up to place where it was standing at the shot, it looked like someone had poured a 1 gallon pail of red paint onto the snow, and the blood trail was 4-6 inches wide all the way to where the deer lay. The bullet had exploded the heart but she still made it 80 yards.
Animals are individuals and some stay on their feet longer than others. Some give no indication that they've been hit, regardless of caliber, while others freak out. This is assuming proper bullet expansion and penetration, and proper shot placement in all cases.
