G36 is about 1.5mm narrower than the standard Glock stuff.
G36 = 28.5mm wide
G20, G21, G29, G30 = 32.5mm wide
Gxx = 30.0mm wide (the standard Glock, G17, G22, G19, etc)
standard Glock, like the 17, 19, 22, etc
http://www.glock.com/english/glock17_tech.htm
slim Glock, only the 36
http://www.glock.com/english/glock36_tech.htm
wide Glocks, the 20, 21, etc
http://www.glock.com/english/glock21_tech.htm
and the Wiki page talking about these, note that they are 10mm and .45ACP calibers only (and the slim is .45ACP):
10mm Auto
The Glock 20, introduced in 1991, was developed for the then-growing law enforcement and security forces market for the 10mm Auto. The pistol will handle both full-power as well as reduced "FBI" loads that have reduced muzzle velocity. Due to the longer cartridge and higher pressures, the pistol is slightly larger than the Glock 17, having an approximately 2.5 mm (0.1 in) greater width and 7 mm (0.3 in) greater length. Though many small parts interchange with the Glock 17, with a close to 50% parts commonality, the major assemblies are scaled-up and do not interchange. The standard magazine capacity of the Glock 20 is 15 rounds. In 2009, Glock announced they would offer a 152 mm (6 inch) barrel as a drop-in option.
The Glock 29 is a 10mm Auto equivalent of the subcompact Glock 26 introduced in 1997 along with the Glock 30. The pistol features a 96 mm (3.8 in) barrel and a standard magazine capacity of 10 rounds. Like other subcompact Glock pistols, the Glock 29 will also function with the factory magazines from its related full-size model, giving an optional capacity of 15 rounds.
.45 ACP
The barrels of all .45 ACP Glock pistols feature octagonal polygonal rifling rather than the hexagonal shaped bores used for models in different chamberings. Octagonal rifling provides a better gas seal in relatively large diameter rifled bores, since an octagon resembles a circle closer than a hexagon.
The Glock 21 is a .45 ACP version of the Glock 20 designed primarily for the American market. Compared to the Glock 20 chambered in 10mm Auto, the slide of the Glock 21 is lighter to compensate for the lower-energy .45 ACP cartridge. The standard Glock 21 magazine is of the single-position-feed, staggered-column type with a capacity of 13 rounds.
The Glock 30 is a .45 ACP version of the subcompact Glock 29, with a standard magazine capacity of 10 rounds. The factory magazine from the Glock 21, with a capacity of 13 rounds, will also function in the Glock 30.
The Glock 36 is a "slimline" version of the subcompact Glock 30 that features an ultra-compact frame and is chambered for the .45 ACP cartridge. The barrel, slide, and magazine are unique to the model. The Glock 36 is the first Glock pistol to be manufactured with a single-stack magazine, having a standard capacity of 6 rounds. Unlike other subcompact Glock pistols, the Glock 36 cannot use factory magazines from its larger relatives due to its single-stack magazine design.
