Motors introduce a lot of changes from your intuition about circuit sizing in general. It has to do with the fact that motors spike the current when they first start rotating (inrush current), but it isn't a sustained current that operates long enough to overheat a wire, busbar, or switch blade. The full load amperes (FLA) and full load current (FLC) values are more representative of what operates in the long term. Those sound like the same term, but there is an essential difference (think "full load actual" and "full load code" to remember it).
When sizing fuses and breakers on the motor circuit, it is important to size it to avoid of nuisance tripping. I.e. tripping under conditions where there is no fault or overload, and it shouldn't trip. Motors loads usually contain their own internal overload protection device built for the motor application anticipating this issue, and the circuit's fuse or breaker is for short-circuit protection. If fuse sizing rules tell you to use a 300A fuse, you can't put a 300A fuse in a 200A disconnect. Therefore, when fused, it would likely have to be in a 400A disconnect.
When unfused, the unfused disconnect just has to be sized for the load, and for any safety factors that apply. An unfused disconnect can only open when manually operated. It doesn't trip, and sizing to avoid nuisance tripping doesn't apply.
