This picture is terribly misleading, and it seems you fell for it yourself. Glock 26 is 1.26" at the widest point of the grip. The Sig is 1.06". This picture shows a difference of > 1/4" between the two grips. The G26 slide is 1" and the P365 is .87". The picture shows the Glock slide sticking out 1/4" further.
I can't quite put my finger on it, but I think it has something to do with the left slide stop being used as a reference point for alignment. All I know is it's bogus because the Glock is NOT 33% wider like the picture basically leads you to believe. Whoever doctored up this photo clearly wanted to bias the results. To make it accurate, the alignment reference point should have been the left side of the widest point of both grips, the left side of the slides, or dead center in the middle of both grips/slides. That would have dramatically changed the outcome of this visual comparison.
Where did this come from?
MLW85547 is correct. It is from Handgun hero.
I stopped using factory measurements long ago because some measured widest point (often controls), some took widest point minus controls (a spot on the grip or slide), some used slide width, and it was a mess.
If the graphic was distorted, it certainly wasn't my intent. But you are absolutely correct. The slide width is 1" on a G26/19/17/34, and I didn't catch that in the graphic. Thanks for noting it, and you are right!
Historically, HandgunHero has matched my overlays very nicely (I used to put guns I had on a X/Y ruler grid, then use the grid to align, then use transparencies, but handgun hero is WAY easier, and they have guns I don't own).
Sig P365 vs Glock 26 pic comparison
From Sigtalk, and courtesy of Cyanide171, see actual photos:
Here is a G27 and G43 compared to a 365:
If the scaling is wrong on HandgunHero, I do apologize. Deception is not my intent, and HandgunHero has proven to be really accurate for other comparisons I have done. Perhaps I should put a G26 on my ruler grid and do my Photoshop comparison overlay. Sadly, I don't have a G26 any longer (I sold mine prior to doing overlays like the one belo, and had to borrow a friend's G43 since I don't own one!).
Regardless, I have held both, and I have a G17 (which shares the slide width of a G26), and the difference is still substantial (if not exactly as pictured by HandgunHero).
I never rely on reported OAW. I either put calipers to mine, or use accurately aligned, overlayed, gridded photographs. I had started using HandgunHero, and the early ones were all perfect. This time, I was too trusting, and will have to pay a lot more attention prior to blindly trusting they aligned it correctly.
The below photo has the identical rulers on the four sides cropped out. This is what I historically did, but was liking the fact HandgunHero saved me tons of photography and photo-editing time!