Does justice require proportionality?

The maxim that "the punishment should fit the crime" tends to go unquestioned in discussions of retributive justice. Those discussions often revolve around whether a particular punishment is proportional or not, rather than taking a step back and looking at the overall concept. In times of war, for example, it often seems necessary to be disproportionate in order to ensure a speedy victory, such as with the nuking of Japan in World War 2. Moreover, disproportionately harsh punishment can be an effective deterrent of crime. So, what is proportionality? And why should the punishment fit the crime?