To some extent, this is the story of firearms. As firearm technology improves, the need to use big, heavy bullets has been reduced.
Looking back to the days of hunting with blackpowder rifles in Africa, it wasn't terribly uncommon for people to be using rifles with ball projectiles that were a significant fraction of a pound in weight. Dr. Samuel Baker used a 2 Bore rifle he referred to as "Baby". A 2 bore rifle shoots a lead ball that weighs 3,500 grains. That's 8 ounces--half a pound. Talk about momentum! Dr. Baker reported that it was not uncommon to get a nosebleed after firing "Baby" due to the tremendous recoil.
Advances in smokeless powder, and then later in bullet technology have improved the situation to where it is not necessary to shoot a half pound projectile with a diameter of 1.33 inches in order to get the desired terminal effect, even on very large African game.
Does that mean that those new rounds are more effective than Baby's half pound lead balls? Probably not, but they are more than adequate and have lots of other advantages.
We're still seeing a progression of technology, albeit much slower than it was in the days of the transition from blackpowder and lead bullets to smokeless powder and jacketed bullets. But technology marches on.
The FBI says the 9mm is the most effective handgun caliber now.
To be fair, that's not what they say. What they say is that, with ammunition currently available, 9mm meets their performance criteria.
The key to understanding this is to understand that their performance criteria creates a pass/fail threshold.
They aren't testing all the available calibers out there to assign them ranks and picking them based on the rank, they are testing the various calibers to see if they pass the performance tests. Any caliber that passes the test is then considered for selection based on the other parameters which are affected by caliber selection. Things like firearm size, agent shooting performance, capacity, training costs, firearm costs, etc.