Each Sweater is unique and one of a kind. They are not standard sizes. The stated sizes are generalization. Considerations for varying sizes needed include - some sweaters are tapered at the bottom, some aren’t. The chest size of the sweater should be 2-4" larger than the size of the person wearing it. The back measurement is from the base of the neck to the bottom of the sweater.
Read about Cowichan knitting.