One of the guys I regularly hunt with likes a 20 for the early teal season. He's deadly with it. No more deadly than he is with his 12, and no more deadly than I am with my 12...but he kills more than his share. That's because if those teal are further away than 30 yards, we just wait for the next batch of birds.
When regular season rolls around, he knows he'll be taking shots at birds that are much more wary, and many of those shots will out closer to 40 yards. That's why he puts the 20 away until next dove season, and brings out the 12. He's smart, he's experienced, and he knows what it takes to reliably kill ducks. That's why I use a 12.
If someone likes to shoot a 20, and his hunting conditions allow the use of lighter payloads, then why not use what he likes?
Our conditions require a bigger gun, with bigger payloads most of the time. That's why I use a 12.
I'd rather just kill that canvasback out there at 40 yards than have to sit and watch it fly by because my gun/payload are insufficient to do the job reliably. That's why I use a 12.
You aren't wrong to be using a 20, but I would be.