In most cases, the heavier caliber handguns are less accurate due to the weight of the bullet, the length of the barrel also reduces accuracy due to the loss in velocity if memory serves.
The Desert Eagle may be a bit different, but ultimately a gun be it a pistol or a rifle, is only as accurate as the person wielding it.
The Desert Eagle is the only hand gun to use a gas piston recoil operated mechanism that is usually reserved for rifles, the reason for this is that the type of round it fires is simply too powerful for conventional recoil operated handguns (This is also the reason the DE is so damn big). The pro's of this design is that the Desert Eagle is able to fire much more powerful rounds usually used in Magnum revolvers, you can see the advantage of having this much firepower in a semi auto platform, the trade off is the size of the weapon itself as well as the weight (It weighs nearly 4 pounds! That's a whole lot of gun) further more, due tot he nature of the gas piston design, it discourages the use of non jacketed rounds, since lead particles that shear off of conventional pistol cartridges can foul the gas tubes and cause malfunctions.
Sorry for the gun nerd thing.
You can read more at wiki
http://en.wikipedia.org/wiki/IMI_Desert_Eagle
EDIT: By the way, he's not shooting the round the pistol is famed for mind you (The .50 Action Express) He's shooting the pistol in .44 Magnum so that also makes a difference