Originally Posted by hi-tek

Could be either clubhead size or Coefficient of Restitution (also known as trampoline effect) or both. Currently the maximum allowed clubhead size is 460cc and the COR limit is .83 per USGA regulations. Imagine a rifle firing a golf ball at the clubface at 100 miles per hour. The ball can not bounce back more than 83 feet and still be in conformance to USGA regulations.