The .30-06 was heavy, kicked like a ticked-off donkey, and wasn't terribly accurate. Factory ammunition wasn't expensive or hard to find, but the rifle's heavy recoil took a good bit of fun out of target practice.
The 7mm-08, on the other hand, is lighter, doesn't kick as hard, gives me sub-minute-of-angle accuracy with factory ammo, and kills deer every bit as quickly and cleanly as did its larger-caliber predecessor.
To me at least, it's more efficient.
So is the bolt-action .22 I now use for squirrels. Like many West Virginians, I grew up hunting squirrels with a shotgun. I started off with a single-shot 20-gauge and later "graduated" to a pump-action 12-gauge.
The 12-gauge killed squirrels quite handily. At ranges of up to 30 yards, it was simply a matter of drawing a bead, squeezing the trigger and watching a lifeless bushytail plummet to the forest floor.
But like the .30-06, the 12-gauge was a load to carry and kicked pretty hard. Its ammo wasn't cheap, either.
The .22, on the other hand, weighs 2 pounds less, has virtually no recoil and uses less expensive ammo. It's dead-on accurate all the way out to 50 yards. To me it feels more efficient.
It's possible to downsize too much, of course.
Hunting grizzly bear with a .243, for example, would be inhumane and foolhardy. The rifle probably would kill the creature, but probably not quickly. And if the bear decided to charge, the .243 would lack the knockdown power needed to plant a ticked-off griz in its tracks.
The key to downsizing is to find just enough firearm to kill critters quickly and cleanly, but at the same time isn't a physical or financial burden to shoot - in other words, enough to open the safe without blowing up the boxcar.
Got that, Butch?