Correct Values of Slide and Frame Widths
G36:
SLIDE 25.5 mm (1.00 in). FRAME 28.0 mm (1.10 in)
G17, 19, 22, 23, 24, 25, 26, 27, 28, 31, 32, 33, 34, 35:
SLIDE 25.5 mm (1.00 in). FRAME 30.0 mm (1.18 in)
G37, G38, G39:
SLIDE 28.5 mm (1.12 in). FRAME 30.0 mm (1.18 in)
G20, 21, 29, 30 (except 30S), 40:
SLIDE 28.5 mm (1.12 in). FRAME 32.5 mm (1.27 in)
why didn't they make the G39 as slim as the G36?
The real question is why 45GAP models did not use a Slide of the same 1-inch width as the G36.
The 45GAP models use the same FRAME as the 9mm, 357SIG, and 40SW models, while maintaining the 1.12-inch SLIDE width of most 45ACP models. Had Glock used the 1-inch slide width of the G36, the result would be near-45ACP power in pistols the exact size of a G17, G19, or G27. Perhaps 20 years ago that would have boosted 45GAP pistol sales and caliber popularity to survivable levels. Most likely not. The 45GAP came (1999) without credentials and will depart leaving no substantive legacy or influence except as a collector's oddity. It is a good round in good pistols that just hasn't had any overwhelming reason for existing. Don't expect Glock to try any more developments of Gaston's shaggy dog. When Gaston goes, no doubt so does the GAP.
width
G36, 1.12"
G39, 1.18"
G27, 1.18"
The values you cite are FRAME widths for the G39 and G27, plus some unknown value of something for the G36. To make an informed comparison, BOTH slide and frame dimensions must be examined. See the opening of this post.
if they wanted to keep the G39 in the same dimensions as G38 and G37, okay, but why don't they make a G39 slim?
A "G39 slim"??? Are you serious? Glock (understandably!) hasn't found it worthwhile to make even a Gen4 version of a G39 or G38. (All that it would take for a Gen4 G39 is substituting the already extant Gen4 Subcompact frame for the G39 Gen3 frame.) What would motivate Glock to tweek the design of such an unpopular model and caliber with a "slim" version?
...since the G36 is so popular...
Whatever gives you that unsupported idea?
The 1999 G36 has never been a popular seller and is rarely held in dealer inventory. Only the related 2013 G30S (G36 slide on a G30SF frame) can be claimed as a popular model, with good justification.
However, the lack of popularity is not really the technical fault of the G36 (after overcoming some early "new model" issues). The real source of G36 unpopularity is its perceived "low capacity" in a market where many purchasers pretend that the gun fights they will have require one or more high-capacity magazines to resolve. (There are no non-LE documented examples of such in the USA.) The G36 is an entirely satisfactory weapon whose only real disadvantage is not being able to use the magazines of any other Glock 45ACP model.
