It really has very little to do with marksmanship. It isn't the size of the target, it is the mobility of the target. Deer and other game don't stand still for long. Since they are prey animals they are always on the lookout. From the time you squeeze the trigger the target may move before the bullet gets there. So you can hold perfectly and still blow the shot due to movement that you have no control over .

I know a little about shooting with more than 25 years active duty, 9 combat deployments and expert qualifications in 7 different weapon platforms. I don't know how many rounds of ammo I have fired in my life but some of the gun shoots I took part in went through 750,000 rounds in 2 days on the range and I shot an M-4 out to 800 yds. There is s reason the military teaches people to chest shoot the enemy. Bigger target and less movement. Period.