Big dogs carry more fat reserves and have a greater heat storage ability than little dogs (surface to volume -ratio)Puppies for instance do not have the body mass to be able to survive rapid dehydration, and adult, or big, dogs tolerate better low temperatures.Small dogs have low body mass that make them lose heat easier. Even the long haired doggies can not adjust their body heat as well as larger breeds.
Small dogs have smaller stomachs and small appetites. You will need to make sure your dog gets the proper nutrition in his small meals. Feed more often (3x per day) and provide high- calorie,with sufficient nutrition.Small dogs need more calories to maintain their body heat: they produce less heat than big dogs, and radiate more of their surface (skin).