It doesn't come down to which one's biggest, oldest, fastest or loudest but which one has the right energy to be the higher ranking dog. Higher ranking dogs are born not made and because it is easier for a dog to follow the more submissive dog will gladly allow the other dog to take the lead.

If you get two dogs that have the same energy levels and are relunctant to back down then that is when trouble begins because they are kept in the same enclosure (house). If they were to be born in the same group in the wild, then one of them would leave to form a new group to avoid conflict.

Sometimes it can just be a territorial thing at which point you as the owner need to step in and remind them that you own the house and they are able to sleep where they do because you allow them to not because that is "their" bed, you allow them to play with your toys and eat your food.

The leader sleeps wherever they want to where as the lower ranking members need to make room and sleep where there is room leftover.