It isn't about killing, it's about stopping and the ability of the shooter. Years ago, more gunowners had an interest in the sport of shooting whether it was hunting or whatever. Today, most people buys guns for the purpose of self defense and rarely even practice with the gun after the first few range sessions. This is even true for police officiers as a recent study indicated that a majority of today's LEO's do not even lists guns or shooting as one of their hobbies. Less interest translates into less practice which translates into poorer shooting ability. I don't know if I believe this but I remember reading on TFL that a DOJ study showed that LEOs missed 92% of the time in actually street shootings.
If your placement is not that good, then a hit with a bigger caliber is better than a hit with a smaller caliber. For example, a hit to the shoulder with a .45ACP will do more damage than a hit to the EXACT SAME area with a .25ACP.
Other factors to consider is the mental state of criminals. Years ago it was ... "I've been shot (I'm already at a disadvantage). I better give up now before I am shot again or even killed" ... while now it's more like ... "If I go, I'm taking everybody with me" ... meaning that criminals do not give up so easily now a days out of fear of being shot. At to that the fact that some have their minds clouded by drugs and might not feel any pain when a bullet strikes them translates into stronger firepower being required to get the job done that was previous done by a weaker caliber.
Finally, each generation is bigger & stronger (physically speaking) than the generation that came before it due to advances in medicine, exercise, lifestyle, etc. and let's not forget that serious criminals are resorting more & more to bullet proof vests and similar things.