Vanity sizing has radically changed the meaning of a size 8 over time, as the chart below shows.
Studies have shown that shoppers prefer to buy clothing labeled with small sizes because it boosts their confidence. So as the weight of the average American woman rose, from 140 lb. in 1960 to 168.5 lb. in 2014, brands adjusted their metrics to help more of us squeeze into more-desirable sizes (and get us to buy more clothes), as TIME reported in a feature this week.
Over time this created an arms race, and retailers went to extremes trying to one-up one another. By the late 2000s, standard sizes had become so forgiving that designers introduced new ones (0, 00) to make up the difference.