Both too little and too much exercise cause welfare harm in dogs, and getting the balance right depends on breed, age, health status and individual needs that owners must learn to read.
Exercise meets multiple welfare needs simultaneously: physical health, mental stimulation, social interaction, and behaviour expression. Getting this balance wrong in either direction causes suffering. Over-exercised puppies develop joint problems. Under-exercised dogs develop anxiety and destructive behaviour. Individual variation requires owners to be observant and responsive rather than following breed templates rigidly.