Brad,
Here's one "gunwriter" who doesn't say bigger bulls are harder to kill--and I've also known other writers who don't believe it either, some of whom have killed more elk than many other hunters and gunwriters. Bob Milek killed a pile of elk, and among his favorite rounds was the .25-06.
That also jibes with a Colorado elk outfitter I met a number of years ago, whose clients had taken an average of around 30 elk a season for many years. Have told this story before but here it is again: I asked him what he felt was a good minimum cartridge for elk hunting, and he immediately said: "Two fifty-seven." I then asked him whether he meant the .257 Roberts or .257 Weatherby, and he said, "Either one!" Bob Milek's .25-06 falls right between them.
As you pointed out, one of the potential problems of judging "adequate" cartridges is is "confirmation bias." Quite a few hunters start with their mind made up. They also rarely or observe other people shooting big game, particularly elk hunters, because they tend to hunt alone even if they camp with others. And even if they do see others kill game, firmly believe a hunter has to personally pull the trigger for rifle/cartridge experience to be valid.
The other problem, of course, is that many hunters (and even many guides and outfitters) still have almost no clue about the difference the specific bullet can make in on-game performance. I still run into many Montanans who firmly believe any bullet lighter than 180 grains is inadequate for elk, both because all they've used are typical cup-and-cores, and they've rarely seen anybody else kill elk. I even knew a younger elk guide in Idaho who believed the .338 Winchester with a 225-grain bullet was the best minimum. Why? Because that's the cartridge he picked years earlier as a teenager--and used with a particular 225-grain cup-and-core. He was even astounded when any smaller cartridge killed big game quickly. Thus, once again, confirmation bias.
Of course, one of the reasons I wrote this is the numerous times I've heard, "Gunwriters always say X." Gunwriters do NOT all have the same opinions, any more than other hunters so do not all write the same things. Perhaps the most blatant example occurred when Eileen and I had a table at the Bozeman gun show maybe a decade ago, partly to sell our own books. One guy around 60 stopped and looked them over, then said, "So you write about hunting rifles? Why is it all you gunwriters say the .270 isn't enough for African plains game?"
I looked at him and said, "I've never written any such thing, because I know from experience it's not true." But he was already too far into his rant to hear me. (Or maybe he was simply a deaf older shooter.) He went on for a couple more minutes before finally leaving. Instead of trying to argue with him, I just waited him out, which over the years I've discovered is easier than arguing with such people.
Personally, the minimum cartridge I know is adequate for elk is, like my outfitter acquaintance in Colorado, the .257, whether Roberts or Weatherby, because I've seen the job done cleanly with one bullet from both.
