The Downsizing is the name for the Sixty Minute War that was used in Fever Crumb' s time, before the exact cause of the event was known. The idea that the great cataclysm was caused by war was only one of several competing theories; it was thought that plagues, earthquakes, or divine wrath may have been other possible causes of the collapse of the Ancient civilization. Fever herself did not believe that war was a likely cause, thinking that the Ancients were too wise to destroy themselves in this way.
Presumably, at some point of time between the start of the Traction Era and the events surrounding the MEDUSA incident, new historical or archaeological evidence was uncovered, and it became generally known that the cause of the Downsizing was the Sixty Minute War. The term "Downsizing" seems to have fallen into disuse after this point.