Shot load depends on your use. For 12 gauge skeet shooters, I’d say that most of them I know use 7/8 oz or 1 oz loads. They seem to work just fine in breaking birds, and they are a lot easier on the shoulder after firing 4 or more rounds (25 shots/round) of skeet. It’s not at all unusual for some skeet shooters to do as well, or better, in the 20 gauge stage, as they do with a 12 gauge. In fact, I had one friend that used a 20 gauge in the 12 gauge stage for just that reason. So within limits, shot mass is not a critical variable in skeet due to the close ranges involved.
In my case, as I take blood anticoagulants for a heart condition, shooting any load over one ounce causes me extensive shoulder bruising after just a few shots. So I have to go with light loads with less shot. A secondary reason for lighter shot loads is with the price of lead shot being what it is, if you are a reloader, your money goes substantially further (or more bangs for your buck).
For hunting other than rabbits, squirrels, and doves, more shot weight is usually called for. But even on a good day, most hunters won’t fire nearly as many shells as a clay bird shooter.
By the way, don’t (didn’t) some european shotshells use square shot for spreader loads?