Also, in the interest of flat trajectory, smaller diameter spitzer bullets became popular over 100 years ago.
The .45-70 black powder cartridge from 135 years ago is probably a better game killer than anything short of the DG rounds TehK1w1 listed above. Yes, better. I've seen it do amazing things.
The problem with it is that its trajectory is anything but flat and it's hard to hit anything past 150 yards, without a good deal of practice. What you hit, will go down, but its hard to hit anything when bullet drop is measured in feet, not inches!
When the US military went to the .30-40 and the .30-06 soon afterward, the objective was a flat trajectory. When hunters adopted the .30-30 and the .30-06, they wanted the same thing. Higher velocities from smokeless powder made these rounds useful, and smaller bullets helped them shoot flatter.
The real reason that the larger bullets are not so popular is that most everyday hunters shoot medium sized game at longer ranges (e.g. deer at 200+ yards) as opposed to large game at shorter ones (e.g. cape buff charging a few yards away).