Larry, Most of Accucraft/AMS product that is modeled in 1:20.3 is based on D&RGW prototype equipment. I believe the D&RGW used standard guage couplers on all there equipment both standard guage & narrow guage. This is why Accucrafts couplers seem oversized. Imagine what the coupler that Accucraft uses would look like on a 1:20.3 scaled std guage piece of rolling stock. I would imagine like me you would view the coupler as being much smaller in proportion to the rest of the car.
Glen is correct. The D&RGW used full-sized couplers on their narrow gauge equipment, as did a fair number of other narrow gauge lines, primarily western ones, though there was no real pattern in that regard. Other narrow gauge lines (particularly eastern ones) used a 3/4-sized coupler.
Here's a shot showing the two sizes right next to each other. This shot is of a standard gauge dump car in the EBT's Mt. Union yards, which were switched by both standard gauge and narrow gauge locomotives--hence both couplers. The D&RGW had a similar arrangement on their "idler flats" that went between their narrow gague locos and standard gauge cars--the difference being that both couplers would be the same (larger) size.
I provided the original measurments for to Accucraft for the couplers. The sample I used was on a car in the Chama yards; not only were dimensions provided, but photos of all sides with a known dimension object (a credit card!) were also provided. I hope that this helps! The best New Year to you and our better half.