Why is driver head size limited?
Driver heads are measured in volume and are limited to 460 cubic centimetres (cc). This limit is enforced to stop technology diminishing the skill level needed to play well off the tee.

What are the benefits of larger heads?
Playing a 460cc driver, like the Titleist 917 D2, gives you the biggest head possible, which means you can miss the middle of the face now and then and still hit a good shot because they tend to be extremely forgiving.

What about smaller heads?
Smaller heads, like the 440cc Titleist 917 D3, create less drag as they move through the air, so should give you faster swing speeds, which translate to faster ball speeds and more distance, but are a little less forgiving than a larger 460cc headed driver.