Shoot them in the right place they die promptly. Shoot them in the wrong place they don't. Doesn't really matter what species we're talking about.
I wouldn't say that if one animal goes 100 yards after a shot, while another goes 50 yards that the 100 yarder was any harder to kill. These variables can be just as much about an individual's tenacity as any trait of the species. Very large animals may take a little longer to bleed out, but I wouldn't say that makes them tougher to kill.
