Not much can be done about the numbers Rocketman discusses; the 28ga barrel thickness at the breech to match up to the frame is extra metal and that's all she wrote.
If the 20 ga frame is fairly small and light to start with, the small ga sets work pretty well. My experience is based on a RizziniB O/U multi-barrel set with 29" 28ga barrels. The Rizzini 20 ga frame is pretty svelte AND I can't hit much with a gun that weighs less than about 6.25 Lbs, so it works for me. I tried the scaled frame gun and it was cute as it was light, but I've failed in the past with similar guns and went with 20 ga frame/heavier gun.
If you do go with small ga barrel sets, consider longer small ga barrels that might balance better with the slightly heavier frame.
