True enough. In fact, reality is worse than what I stated above:
The AW IMR 18650 cells claim 1600 mAh.
The AW black 18650 cells claim 2900.
The orange MNKE 26650 cells claim 4000.
For both kinds of AW 18650 cells, this is very close to what I measure on my CellPro charger (e.g. full charge of AW IMR 18650 typically reports 1570 mAh in). For the 26650, I have never seen anything close to 4000 -- best to date is around 2800. They may improve with a few more cycles, but definitely disappointing thus far. And unfortunately there isn't much competition in the 26650 space. In fact, as far as I can tell, there is no such thing as a LiCo 26650.
That is quite revealing. In that case it would be most interesting to know what the difference in capacity is between the AW 18650 3100maH rated cell compared to the best 26650. If there isn't much difference, then you're right that the extra width of the battery tube of the 26650 would be a disadvantage for more compact style lights.