Bigger dogs are not better dogs, Gary.  Hauling freight and heavy loads is a different activity from racing with lightweight materials and hauling comparatively small loads of essential equipment and food.  Resting dogs should always be discretionary and I don't think you want to provide incentive to keep a dog running that should be resting. Limiting the size of the sled and space available for resting dogs might be helpful in guiding drivers to develop a strategy which does not depend upon resting and rotating.  Space would be used for emergency and unexpected situations in which a dog must be temporarily loaded for health and safety concerns.