120V loads can't be considered balanced because you never know which ones will be on. It would be possible (but highly unlikely) that all 120V loads on phase A are on and all on phase B are off. That puts the same current on the feed to A and the neutral. A welder doesn't connect to the neutral. SO if it contributed 30A to the load calc, you could subtract 30A off the neutral current.
Typically people install a neutral that is either the same size as the ungrounded conductors or maybe a size or two smaller. Full size reduces voltage drop and allows for all 120V circuits in the panel. If this panel was supplying something like a bunch of 240V heaters and one or two lighting and receptacle circuits, you could consider dropping the neutral way down. It is a way to save cost, but hampers future plans if they want to repurpose into more 120V loads and less 240V ones. If much of the 200A panel is not yet used (e.g. a load calc of 100A with a feeder rating of 200A), then it would be a better design to assume all future growth is unbalanced 120V loads.
