It isn't quite that simple. A properly made pocket has cushions that extend to a point, then have a facing that forms the inside, ' mouth' of the pocket. Therefore, the with of the pocket is determined by the length of the cushions.
The length or the cushions is determined by the length of the material they glue onto, a part of the rail called the subrail. To properly tighten up a pocket, the subrails need to be lengthened. What that gets is a rail that plays properly along its whole length.
Proper:
Or you can jus lump a bunck of rubber in there, but the cushions will play different in the areas close to the pockets: