When the engine produces a lot of torque, the half-shafts twist like a spring and then release when the tire looses traction.
If the half-shafts are identical in size, the release on one side can cause the other side to wind up and then reverse the process.
Exciting the system in this way ultimately causes the wheels to start bouncing around in a phenomenon known as axle hop. When the wheels start hopping, the car doesn't accelerate and, if you're trying to accelerate out of a corner, you can easily get totally out of control.

Most manufacturers try all kinds of fancy suspension geometries and control schemes to get axle hop under control.

The GM suspension engineers decided to go back and look at the root cause of the problem and discovered that it was triggered by the half-shafts alternately winding up and releasing.

After analyzing the problem, they discovered that by changing the effective spring rate of the axle shafts they could virtually eliminate axle hop by ensuring the oscillation frequency of each side was different, thus eliminating the excitation that was occurring.

They did this by making the left half-shaft almost twice the diameter of the one on the right. The result is that even with all that torque, the 2009 CTS-V has some of the cleanest, smoothest launches with no skittering or bouncing around.
The system works so well that GM has applied for a patent on the design and the Corvette ZR1 uses the same setup.