Glock 20
The Glock model 20 is the full-size 10-mm pistol. The length of the pistol is 7.59 inches and the height is 5.47 inches. The standard magazine of the G20 has a capacity of 15 rounds. The weapon is large and difficult to use for concealed carry. Reducing the size of the grip would leave you the advantage of having the longer barrel while reducing the profile (the bulge produced by a concealed handgun) of the weapon. By reducing the size of the grip you would have the option of using the smaller capacity magazines of the G29 or the full-size magazines of the G20.
Glock 29
The Glock model 29 is a sub-compact 10-mm pistol with a standard magazine capacity of 10 rounds. The G29 is a small framed weapon that's 6.77 inches long and 4.45 inches high. The handgun was designed for concealment and, most likely, would not benefit from a grip reduction. Even with a grip reduction the profile of the weapon would remain the same because the magazine cannot be reduced. The G29 can use the magazines from the larger-sized G20.
Polymer Lower Receivers
The lower receiver, or grip, of all Glock handguns is made from a tough plastic polymer. Glock was the first manufacturer to introduce grips that are nearly completely made of plastic. An advantage of the plastic receiver is that it's lightweight, impervious to rust and, for our purposes, can be shortened with a PVC blade.
Procedure
You can reduce the length of the grip by taking the weapon apart and cutting off the lower portion of the grip. Carefully mark a line for your cut, cut the grip with a PVC blade and sand down the rough edges. Before making your cut, place the grip in a vise (with padded sides) so that you can make a careful and controlled cut.