There are some people that hunt and think, well, I'll only take a perfect shot, and all my shots that I take will be perfect shots, so I only need a .223, or a .243 to do the job. Then, there are people who hunt that think, well, maybe I won't make a perfect shot, or, sometimes the rest won't be the best, or, sometimes the sun/wind/rain will interfere, or sometimes the giant buck will be over 300 yards away, or some other factor will make a less than perfect shot probable.
Now, for all those of you that only make perfect shots, this won't apply, but, for the rest of us, a couple examples make the following clear.
Example 1: A buck is facing a very muddy swamp, maybe 25 yards away. You are pretty close, say 100 yards. A less than perfect shot will make him run, straight for the swamp. I don't know about the rest of you all, but I would prefer to bring him down BEFORE he gets 100 yards deep in the mud, so I'll take the bigger round.
Example 2: It's a windy day. There's a buck 200 yards off. Even a perfect shot will be affected by the wind. I don't know about you all, but I would prefer a heavier round less likely to be blown significantly off course so I don't have to apply as much Kentucky Windage.
Example 3: It is near dark, and the Boone and Crockett Record Beating buck steps out from behind a bush 400 yards away. I don't know about you all, but I would prefer a gun that has enough killing power and doesn't need to be aimed a foot high to do the job.
Example 4: A buck is standing 100 yards away, in the hill country of Texas, with a whole lot of rocks and hills. A less than perfect shot will make him run immediately out of your field of vision. I don't know about you all, but I would prefer the entry and exit wound to be as big as they can be, so I can have the most amount of blood to track by.
So, there are some arguments for bigger rounds for hunting. What are the arguments for the smaller rounds? I mean, like real arguments, not "it's all I need." I only take a few rounds with me, so it can't be ammo weight. Likewise, my deer rifle doesn't weigh much more than my Colt AR, so it's not that either.