Although golf's governing body, the United States Golf Association, has no rule against using a standard grip on a putter, players sometimes use custom-designed putter grips. The length and shape of a putter grip are closely regulated by the USGA; the particulars of its design are specified in the Rules of Golf under Rule 4 – Clubs; Appendix II – Design of Clubs; Category 3 – The Grip.
The USGA regulates the actual thickness of grips, regardless of whether the grip is for a wood, an iron or a putter. The diameter of all grips is limited to 1.75 inches. A grip's diameter cannot exceed this measurement, regardless of the direction in which it is measured. So although putter grips may be larger in diameter than grips of other golf clubs, they don't have to be.
Traditionally, putter grips have been shaped differently from other grips. They used a so-called "pistol-type" shape with a flat spot that helped the player position his hands for the putting stroke, which requires a slightly different hand placement than for regular clubs. The Rules of Golf still allow putter grips to use this traditional shape.
When you toss a ball underhand, the flexing of your relaxed wrist helps you to better judge how far the ball will go. Similarly, a relaxed grip makes it easier to judge the distance a putted ball will roll. Relaxed wrists are also less likely to create unintentional clubface rotation. In short, a relaxed grip improves both line and feel. The thinner your putter grip is, the more tightly some golfer grip it in order to keep control of the putter. A fatter grip allows you to hold on more lightly while still keeping control of the club.
There was a time when, if a player wanted a truly fat grip, he had to create it himself. But that changed when K.J. Choi began winning tournaments with a Tiger Shark SuperStroke Fatso grip beginning in 2007. Choi has won several times on the PGA Tour while using that style of grip, including the 2011 Players Championship. At nearly the maximum allowed diameter, it was twice the size of most other available grips. Choi's success created a market for bigger putter grips.