Detroit has not had the largest peak-to-trough decline in percentage terms among American large cities

Although it is getting close.  Detroit has lost about 58 percent of its 1950 population; St. Louis has lost about 59 percent.  And Detroit's population is much bigger than it was in 1900; St. Louis has lost about 30 percent of its population since 1900 (just prior to the 1904 World's Fair, when 20 million people visited St. Louis).