If you want to split hairs you could say that one ounce of shot is an under load in the 12, just right in the 16 and too heavy for the 20.

My own hunting showed that the 20 was a poor choice for most ducks and geese but was fine for most of the smaller birds. The 12 was fine for ducks and geese but was a little over kill for some of the smaller birds. Like most compromises the 16 was in between. Back years ago when lead was king, I shot several hundred ducks with one ounce in my trusty Winchester Model 12 in 28 gauge not a load that I would recommend but I shot what I had.

If you are a great shot or shoot under the right conditions nothing makes a real difference. If you are looking for a magic bullet there are none. It get harder to get the same pattern out of smaller gauges due to long shot coulum and bore contact causing more pellets to be deformed. But if you are within 12" plus or minus from the center of your pattern all three loads will kill almost every time. In fact if you shoot that well a .410 with a half ounce of shot will do fine. In the end shoot what fits you and what you shoot well. The rest will take care of itself.