So I have been carrying AIWB for over 9 months and have noticed something fairly interesting.
My two carry options are a fullsize USP .45 in a TRex Raptor type holster with claw and a G30s in a TRex Raptor with the claw installed.  I also carry a spare mag on my weak side in an IWB kydex carrier.

My go to is the USP for several reasons...I like a fullsize gun.  I shoot it better than the G30s.  But one of the biggest reasons is that it is way more comfortable, concealable and secure.  
For some reason, the grip sticks out way less than the G30s and the gun stays exactly where I put it.  It does not move or shift hardly at all and conceals unbelievably well.
The G30s which is much smaller has a bad habit of tilting forward and canting inward.  This causes it to be more uncomfortable and less concealable.  

I'm not saying this is fact but my suspicions are that the longer barrel and holster of the USP cause it to be more stable and keep the grip and gun in place.  So the question would be do you think if the holster maker extended the holster an inch or so on shorter barreled guns,  would it be more stable?
Picture a G19 in a G17 holster or maybe my G30s in a G21 holster (although that may not work due to the different slide profile).  Or do you think the weight of the longer barrel is what helps to counter balance the weight of the mag/grip?  Or a combo of the barrel weight and length and longer holster working together to stabilize things?

Let's discuss...

What kind of belt do you wear?

This is sometimes true, depending on the gun and on your body build. Carrying a Glock 26 in a Glock 19 holster can at times tuck the gun more making it more concealed. The trade off may be in comfort.

Depends...most of the time I'm using the double layer 5.11 belt.
When dressed up I have the Daltech Force Bio belt.

I've found that a longer AIWB holster conceals better especially with some sort of wedge at the bottom. In fact, several holster makers incorporate a sewn, molded, or add-on wedge of some sort.

I have noticed the canting outward issue with several of my belts while using a holster intended for a G26 whereas the G17 doesn't suffer from that issue.  I think the combination of heavy butt with short barrel makes it more susceptible to canting out as neither my PF-9 nor my 442 suffer from the problem.

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Not only have I been told this by different manufacturers, I have also experienced this.  If there's no wedge, you're handicapping yourself.  AIWB-CDA II fully adjustable.

G17 length holsters work best for me. G34 holsters are just barely too long and G19 holsters are too short.

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I carry a G19 in a JM Custom AIWB. It's comfortable for me. I have a short torso and wear the holster at "low ride." I'm 5'9" if that helps. The grip prints ever so slightly but that's just because I have a little more belly to lose. It conceals remarkably well, and I can wear it all day it is so comfortable (I did order it with the extra tuck option which is a molded wedge on the bottom of the holster intended to cant the muzzle outward and the grip inward). I don't know that I would have preferred a G17 size holster.

I do remember, back in the day, getting an M&P in a trade that came with a JM Custom appendix holster. I had never worn appendix before and was skeptical of that position having been used to OWB/IWB at the 3:30-4:30 area. I ended up selling it, but I did try it on once and was impressed at how comfortable it was to wear. I'm not sure how much the length played a part in my perception or if it was just the fact that the holster is very well made.

Now, I can't imagine carrying anywhere else except AIWB. I bought a G19 sized holster in case I plan to add a G26 at some point. I didn't order it for a G17 because I didn't think, and still don't, that I would ever carry a G17 in the appendix position.

I've had similar observations. The shorter barrels and heavier grips with full mags does in fact make the gun top heavy and feel like it wants to roll out of the waistband. It doesn't feel like it actually will, just that it kind of feels sloppy and pushed away from my torso.

I carry a G26 and J-Frames. It's not the case at all with the j-frames btw.

I ended up getting a G19 sized CCC Shaggy for my G26 for the benefit of the greater length (and I have a G19 too). I used to have a G26 sized AIWB holster that felt like I described above. The 19 sized holster stays nice and secure in the sweet spot with just that little extra leverage from it's length.

Physique does matter too. I've gotten a little out of shape and having just the slightest belly makes wearing AIWB less comfortable and prints a bit more. I can only imagine it would be worse with a shorter holster.

Cheers!
-JC

IMO there is a trade off. The shorter barrel guns will have a tendency to push away from the body on the topside of the waistband because there is more pressure pressing it away than there is barrel to even out those forces and keep the grip tucked tightly to the body. At the same time, most people find it more comfortable to AIWB a shorter/smaller gun because then the longer slide length isn't digging into their soft tissue or upper area of the leg.

I daily carry a G19 in a holster designed for a 19 and for me its the perfect balance. I may could see a benefit of getting a 19 holster for your 26 because it would ride a little better, but going any longer than a fullsize holster when it isn't necessary is not practical