Shot size is all about tradeoff's between pattern density and shot size.
For example, there are a lot more #9 shot in a target load than there are buckshot in a buckshot load. So, at any given range, with everything else being equal, the #9 shot will give you better target density than the buckshot.
However, all things are never equal. Choke size, target size, your particular shotgun will pattern differently than the next. It's never about spread (whatever that is). What's important is how many shot you can put into the target and is the shot large enough to kill the target. You've got to match the shot size to the game sought, and the pattern density has to be enough to put multiple hits on the target.
Back in the day I used lead #6 for decoying ducks and lead #4 for decoying geese. Then the gummint made us switch to steel shot and I had problems knocking down ducks with steel #6 for a variety of reasons. Steel isn't as heavy as lead, steel patterns differently than lead, steel doesn't penetrate as well as lead, etc, etc. So, I had to change my way of thinking about how shotguns operate against wildfowl.
The best idea is to take your shotgun to an open field and pattern it with several loads. Take a big piece of paper, trace a duck outline on it, and shoot it at 40 yards. Count the hits. Do this several times with different loads and you'll learn what your shotgun likes.