Mariah Chase

It's a big market opportunity with growing sales potential, but retailers can't seem to figure it out. The plus-size apparel market generated $17.5 billion in sales between May 2013 and April 2014, up 5 percent from the year prior, according to market research firm The NPD Group. But that figure likely under-represents the true opportunity of the demographic given that the average American woman is a size 14, according to the Centers for Disease Control and Prevention.