Everthing else being equal, the 12 would have a superior pattern to the 16, and the 16 would be superior to the 20. The 20 would suffer some deformed pellets at the bottom due to the shot column being longer and skinnier. Ballisticians refer to this as effiency, and a 20 with an oz isn't considered the model of effiency.

Now, go work on getting everything equal! Did I mention shooting 1 oz loads in a 20 for any length of time is usually pretty unpleasant?
Best,
Ted