wait, so you never use 3.5"?
I don't get out to hunt too often and I seldom get out 3.5" shells, but theoretically, I can think of 3 reasons for 3.5" shells:
1. The bird continues its forward momentum as the string of shot makes impact. The shot cloud is longer and will therefore cover a greater area of your target as momentum carries the target forward and BBs continue to impact.
2. Your shot will be traveling 600-750 mph (depending on distance) while the bird is going 10-30mph. If broader impact is invalid due to the high rate of shot speed (closer shots) it only means then that more shot is impacting the same area, JUST LIKE A 10ga. You effectively have a 10ga at closer ranges.
3. Choke selection notwithstanding, a longer shot string at farther distances is more forgiving of imprecise shots. Birds flying away or shot clouds placed to the right or left of the body leaves trailing shot to snag a wing or for the bird to fly into if you get out ahead of it too far. (It's safe to say I don't have a problem leading moving targets too far out.)
1. No, I never use 3.5", don't own a shotgun that'll even chamber them.
2. (you're #1), that's making the assumption that you've actually lead the bird far enough for your shot string to be in front of it. The vast majority of shotgun misses are BEHIND the intended target so a longer shot string means you miss more further behind.
3. Do you want 5 check out lanes w/ 6 people in line open @ Fleet Farm on Black Friday, or all 30 with no line? 10ga and it's larger bore diameter delivers a shorter and more densely packed shot string upon the target in a shorter time period.
4. You're missing further behind again. Shooting an extra 1/2" of shot and praying for shit-assed luck is hardly justification for that extra 1/2".
So in summary, lead the bird, lead the bird, lead the bird, and if that ain't workin', use a 10ga and lead the fuggin' bird.