By comparison , carom billiard balls are generally 2.375 i , or 61.5 m. While UMB , the International Olympic Committee-recognized world carom billiards authority , permits balls as small as 61.0 m , no major manufacturer produces such balls any longer . ( By comparison Carom billiard balls are generally 2.375 i , or 61.5 m. While UMB , the International Olympic Committee-recognized world carom billiards authority , permits balls as small as 61.0 m , no major manufacturer produces such balls any longer , and the de facto standard is 61.5 m. Modern coin operated pool tables generally use one of three methods to return the cue ball to the front of the table while the numbered balls return to an inaccessible area : the cue ball is larger than the other balls or heavier , or has a magnetic core .